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This is the Reference Manual for the MySQL Database System, version
5.0, up to release 5.0.23. It is not intended for use with older
versions of the MySQL software due to the many functional and other
differences between MySQL 5.0 and previous versions.
If you are using an earlier release of the MySQL software, please refer
to the `MySQL 3.23, 4.0, 4.1 Reference Manual', which provides coverage
of the 3.22, 3.23, 4.0, and 4.1 series of MySQL software releases.
Differences between minor versions of MySQL 5.0 are noted in the
present text with reference to release numbers (5.0.X).
* Menu:
* introduction:: General Information
* installing:: Installing and Upgrading MySQL
* tutorial:: Tutorial
* using-mysql-programs:: Using MySQL Programs
* database-administration:: Database Administration
* replication:: Replication
* optimization:: Optimization
* client-utility-programs:: Client and Utility Programs
* language-structure:: Language Structure
* charset:: Character Set Support
* data-types:: Data Types
* functions:: Functions and Operators
* sql-syntax:: SQL Statement Syntax
* storage-engines:: Storage Engines and Table Types
* ndbcluster:: MySQL Cluster
* spatial-extensions:: Spatial Extensions
* stored-procedures:: Stored Procedures and Functions
* triggers:: Triggers
* views:: Views
* information-schema:: The `INFORMATION_SCHEMA' Database
* precision-math:: Precision Math
* apis:: APIs and Libraries
* connectors:: Connectors
* extending-mysql:: Extending MySQL
* problems:: Problems and Common Errors
* error-handling:: Error Codes and Messages
* credits:: Credits
* news:: MySQL Change History
* porting:: Porting to Other Systems
* environment-variables:: Environment Variables
* regexp:: Regular Expressions
* limits:: Limits in MySQL
* restrictions:: Feature Restrictions
* gpl-license:: GNU General Public License
* mysql-floss-license-exception:: MySQL FLOSS License Exception
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1 General Information
*********************
* Menu:
* manual-info:: About This Manual
* manual-conventions:: Conventions Used in This Manual
* what-is-mysql-ab:: Overview of MySQL AB
* what-is:: Overview of the MySQL Database Management System
* maxdb:: Overview of the MaxDB Database Management System
* roadmap:: MySQL Development Roadmap
* information-sources:: MySQL Information Sources
* bug-reports:: How to Report Bugs or Problems
* compatibility:: MySQL Standards Compliance
The MySQL(R) software delivers a very fast, multi-threaded, multi-user,
and robust SQL (Structured Query Language) database server. MySQL
Server is intended for mission-critical, heavy-load production systems
as well as for embedding into mass-deployed software. MySQL is a
registered trademark of MySQL AB.
The MySQL software is Dual Licensed. Users can choose to use the MySQL
software as an Open Source product under the terms of the GNU General
Public License (`http://www.fsf.org/licenses/') or can purchase a
standard commercial license from MySQL AB. See
`http://www.mysql.com/company/legal/licensing/' for more information on
our licensing policies.
The following list describes some sections of particular interest in
this manual:
* For a discussion about the capabilities of the MySQL Database
Server, see *Note features::.
* For installation instructions, see *Note installing::. For
information about upgrading MySQL, see *Note upgrade::.
* For information about configuring and administering MySQL Server,
see *Note database-administration::.
* For information about setting up replication servers, see *Note
replication::.
* For tips on porting the MySQL Database Software to new
architectures or operating systems, see *Note porting::.
* For a tutorial introduction to the MySQL Database Server, see
*Note tutorial::.
* For benchmarking information, see the `sql-bench' benchmarking
directory in your MySQL distribution.
* For a history of new features and bugfixes, see *Note news::.
* For a list of currently known bugs and misfeatures, see *Note
bugs::.
* For future plans, see *Note roadmap::.
* For a list of all the contributors to this project, see *Note
credits::.
*Important*:
To report errors (often called `bugs'), please use the instructions at
*Note bug-reports::.
If you have found a sensitive security bug in MySQL Server, please let
us know immediately by sending an email message to <security AT mysql.com>.
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1.1 About This Manual
=====================
This is the Reference Manual for the MySQL Database System, version
5.0, through release 5.0.23. It is not intended for use with older
versions of the MySQL software due to the many functional and other
differences between MySQL 5.0 and previous versions.
If you are using a version 4.1 release of the MySQL software, please
refer to the `MySQL 3.23, 4.0, 4.1 Reference Manual', which covers the
3.23, 4.0, and 4.1 series of MySQL software releases. Differences
between minor versions of MySQL 5.0 are noted in the present text with
reference to release numbers (5.0.X).
Because this manual serves as a reference, it does not provide general
instruction on SQL or relational database concepts. It also does not
teach you how to use your operating system or command-line interpreter.
The MySQL Database Software is under constant development, and the
Reference Manual is updated frequently as well. The most recent version
of the manual is available online in searchable form at
`http://dev.mysql.com/doc/'. Other formats also are available there,
including HTML, PDF, and Windows CHM versions.
The Reference Manual source files are written in DocBook XML format.
The HTML version and other formats are produced automatically,
primarily using the DocBook XSL stylesheets. For information about
DocBook, see `http://docbook.org/'
The DocBook XML sources of this manual are available from
`http://svn.mysql.com/svnpublic/mysqldoc/'.
If you have any suggestions concerning additions or corrections to this
manual, please send them to the documentation team at <docs AT mysql.com>.
This manual was originally written by David Axmark and Michael `Monty'
Widenius. It is maintained by the MySQL Documentation Team, consisting
of Paul DuBois, Stefan Hinz, Mike Hillyer, and Jon Stephens. For the
many other contributors, see *Note credits::.
The copyright to this manual is owned by the Swedish company MySQL AB.
MySQL(R) and the MySQL logo are registered trademarks of MySQL AB.
Other trademarks and registered trademarks referred to in this manual
are the property of their respective owners, and are used for
identification purposes only.
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1.2 Conventions Used in This Manual
===================================
This manual uses certain typographical conventions:
* `Text in this style' is used for SQL statements; database, table,
and column names; program listings and source code; and
environment variables. Example: `To reload the grant tables, use
the `FLUSH PRIVILEGES' statement.'
* Text in this style indicates input that you type in examples.
* `Text in this style' indicates the names of executable programs
and scripts, examples being `mysql' (the MySQL command line client
program) and `mysqld' (the MySQL server executable).
* TEXT IN THIS STYLE is used for variable input for which you should
substitute a value of your own choosing.
* Filenames and directory names are written like this: `The global
`my.cnf' file is located in the `/etc' directory.'
* Character sequences are written like this: `To specify a wildcard,
use the ‘`%'’ character.'
* _Text in this style_ is used for emphasis.
* *Text in this style* is used in table headings and to convey
especially strong emphasis.
When commands are shown that are meant to be executed from within a
particular program, the prompt shown preceding the command indicates
which command to use. For example, `shell>' indicates a command that
you execute from your login shell, and `mysql>' indicates a statement
that you execute from the `mysql' client program:
shell> type a shell command here
mysql> type a mysql statement here
The `shell' is your command interpreter. On Unix, this is typically a
program such as `sh', `csh', or `bash'. On Windows, the equivalent
program is `command.com' or `cmd.exe', typically run in a console
window.
When you enter a command or statement shown in an example, do not type
the prompt shown in the example.
Database, table, and column names must often be substituted into
statements. To indicate that such substitution is necessary, this
manual uses DB_NAME, TBL_NAME, and COL_NAME. For example, you might see
a statement like this:
mysql> SELECT COL_NAME FROM DB_NAME.TBL_NAME;
This means that if you were to enter a similar statement, you would
supply your own database, table, and column names, perhaps like this:
mysql> SELECT author_name FROM biblio_db.author_list;
SQL keywords are not case sensitive and may be written in any
lettercase. This manual uses uppercase.
In syntax descriptions, square brackets (‘`['’ and ‘`]'’)
indicate optional words or clauses. For example, in the following
statement, `IF EXISTS' is optional:
DROP TABLE [IF EXISTS] TBL_NAME
When a syntax element consists of a number of alternatives, the
alternatives are separated by vertical bars (‘`|'’). When one
member from a set of choices _may_ be chosen, the alternatives are
listed within square brackets (‘`['’ and ‘`]'’):
TRIM([[BOTH | LEADING | TRAILING] [REMSTR] FROM] STR)
When one member from a set of choices _must_ be chosen, the
alternatives are listed within braces (‘`{'’ and ‘`}'’):
{DESCRIBE | DESC} TBL_NAME [COL_NAME | WILD]
An ellipsis (`...') indicates the omission of a section of a statement,
typically to provide a shorter version of more complex syntax. For
example, `INSERT ... SELECT' is shorthand for the form of `INSERT'
statement that is followed by a `SELECT' statement.
An ellipsis can also indicate that the preceding syntax element of a
statement may be repeated. In the following example, multiple
RESET_OPTION values may be given, with each of those after the first
preceded by commas:
RESET RESET_OPTION [,RESET_OPTION] ...
Commands for setting shell variables are shown using Bourne shell
syntax. For example, the sequence to set the `CC' environment variable
and run the `configure' command looks like this in Bourne shell syntax:
shell> CC=gcc ./configure
If you are using `csh' or `tcsh', you must issue commands somewhat
differently:
shell> setenv CC gcc
shell> ./configure
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1.3 Overview of MySQL AB
========================
MySQL AB is the company of the MySQL founders and main developers.
MySQL AB was originally established in Sweden by David Axmark, Allan
Larsson, and Michael `Monty' Widenius.
We are dedicated to developing the MySQL database software and
promoting it to new users. MySQL AB owns the copyright to the MySQL
source code, the MySQL logo and (registered) trademark, and this
manual. See *Note what-is::.
The MySQL core values show our dedication to MySQL and Open Source.
These core values direct how MySQL AB works with the MySQL server
software:
* To be the best and the most widely used database in the world
* To be available and affordable by all
* To be easy to use
* To be continuously improved while remaining fast and safe
* To be fun to use and improve
* To be free from bugs
These are the core values of the company MySQL AB and its employees:
* We subscribe to the Open Source philosophy and support the Open
Source community
* We aim to be good citizens
* We prefer partners that share our values and mindset
* We answer email and provide support
* We are a virtual company, networking with others
* We work against software patents
The MySQL Web site (`http://www.mysql.com/') provides the latest
information about MySQL and MySQL AB.
By the way, the `AB' part of the company name is the acronym for the
Swedish `aktiebolag,' or `stock company.' It translates to `MySQL,
Inc.' In fact, MySQL, Inc. and MySQL GmbH are examples of MySQL AB
subsidiaries. They are located in the United States and Germany,
respectively.
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1.4 Overview of the MySQL Database Management System
====================================================
* Menu:
* history:: History of MySQL
* features:: The Main Features of MySQL
* stability:: MySQL Stability
* table-size:: How Large MySQL Tables Can Be
* year-2000-compliance:: Year 2000 Compliance
MySQL, the most popular Open Source SQL database management system, is
developed, distributed, and supported by MySQL AB. MySQL AB is a
commercial company, founded by the MySQL developers. It is a second
generation Open Source company that unites Open Source values and
methodology with a successful business model.
The MySQL Web site (`http://www.mysql.com/') provides the latest
information about MySQL software and MySQL AB.
* MySQL is a database management system.
A database is a structured collection of data. It may be anything
from a simple shopping list to a picture gallery or the vast
amounts of information in a corporate network. To add, access, and
process data stored in a computer database, you need a database
management system such as MySQL Server. Since computers are very
good at handling large amounts of data, database management
systems play a central role in computing, as standalone utilities,
or as parts of other applications.
* MySQL is a relational database management system.
A relational database stores data in separate tables rather than
putting all the data in one big storeroom. This adds speed and
flexibility. The SQL part of `MySQL' stands for `Structured Query
Language.' SQL is the most common standardized language used to
access databases and is defined by the ANSI/ISO SQL Standard. The
SQL standard has been evolving since 1986 and several versions
exist. In this manual, `SQL-92' refers to the standard released in
1992, `SQL:1999' refers to the standard released in 1999, and
`SQL:2003' refers to the current version of the standard. We use
the phrase `the SQL standard' to mean the current version of the
SQL Standard at any time.
* MySQL software is Open Source.
Open Source means that it is possible for anyone to use and modify
the software. Anybody can download the MySQL software from the
Internet and use it without paying anything. If you wish, you may
study the source code and change it to suit your needs. The MySQL
software uses the GPL (GNU General Public License),
`http://www.fsf.org/licenses/', to define what you may and may not
do with the software in different situations. If you feel
uncomfortable with the GPL or need to embed MySQL code into a
commercial application, you can buy a commercially licensed
version from us. See the MySQL Licensing Overview for more
information (`http://www.mysql.com/company/legal/licensing/').
* The MySQL Database Server is very fast, reliable, and easy to use.
If that is what you are looking for, you should give it a try.
MySQL Server also has a practical set of features developed in
close cooperation with our users. You can find a performance
comparison of MySQL Server with other database managers on our
benchmark page. See *Note mysql-benchmarks::.
MySQL Server was originally developed to handle large databases
much faster than existing solutions and has been successfully used
in highly demanding production environments for several years.
Although under constant development, MySQL Server today offers a
rich and useful set of functions. Its connectivity, speed, and
security make MySQL Server highly suited for accessing databases
on the Internet.
* MySQL Server works in client/server or embedded systems.
The MySQL Database Software is a client/server system that
consists of a multi-threaded SQL server that supports different
backends, several different client programs and libraries,
administrative tools, and a wide range of application programming
interfaces (APIs).
We also provide MySQL Server as an embedded multi-threaded library
that you can link into your application to get a smaller, faster,
easier-to-manage standalone product.
* A large amount of contributed MySQL software is available.
It is very likely that your favorite application or language
supports the MySQL Database Server.
The official way to pronounce `MySQL' is `My Ess Que Ell' (not `my
sequel'), but we don't mind if you pronounce it as `my sequel' or in
some other localized way.
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1.4.1 History of MySQL
----------------------
We started out with the intention of using the `mSQL' database system
to connect to our tables using our own fast low-level (ISAM) routines.
However, after some testing, we came to the conclusion that `mSQL' was
not fast enough or flexible enough for our needs. This resulted in a
new SQL interface to our database but with almost the same API
interface as `mSQL'. This API was designed to allow third-party code
that was written for use with `mSQL' to be ported easily for use with
MySQL.
The derivation of the name MySQL is not clear. Our base directory and a
large number of our libraries and tools have had the prefix `my' for
well over 10 years. However, co-founder Monty Widenius's daughter is
also named My. Which of the two gave its name to MySQL is still a
mystery, even for us.
The name of the MySQL Dolphin (our logo) is `Sakila,' which was chosen
by the founders of MySQL AB from a huge list of names suggested by
users in our `Name the Dolphin' contest. The winning name was submitted
by Ambrose Twebaze, an Open Source software developer from Swaziland,
Africa. According to Ambrose, the feminine name Sakila has its roots in
SiSwati, the local language of Swaziland. Sakila is also the name of a
town in Arusha, Tanzania, near Ambrose's country of origin, Uganda.
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1.4.2 The Main Features of MySQL
--------------------------------
The following list describes some of the important characteristics of
the MySQL Database Software. See also *Note roadmap::, for more
information about current and upcoming features.
Internals and Portability:
* Written in C and C++.
* Tested with a broad range of different compilers.
* Works on many different platforms. See *Note which-os::.
* Uses GNU Automake, Autoconf, and Libtool for portability.
* APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby, and Tcl
are available. See *Note apis::.
* Fully multi-threaded using kernel threads. It can easily use
multiple CPUs if they are available.
* Provides transactional and non-transactional storage engines.
* Uses very fast B-tree disk tables (`MyISAM') with index
compression.
* Relatively easy to add other storage engines. This is useful if
you want to add an SQL interface to an in-house database.
* A very fast thread-based memory allocation system.
* Very fast joins using an optimized one-sweep multi-join.
* In-memory hash tables, which are used as temporary tables.
* SQL functions are implemented using a highly optimized class
library and should be as fast as possible. Usually there is no
memory allocation at all after query initialization.
* The MySQL code is tested with Purify (a commercial memory leakage
detector) as well as with Valgrind, a GPL tool
(`http://developer.kde.org/~sewardj/').
* The server is available as a separate program for use in a
client/server networked environment. It is also available as a
library that can be embedded (linked) into standalone
applications. Such applications can be used in isolation or in
environments where no network is available.
Data Types:
* Many data types: signed/unsigned integers 1, 2, 3, 4, and 8 bytes
long, `FLOAT', `DOUBLE', `CHAR', `VARCHAR', `TEXT', `BLOB', `DATE',
`TIME', `DATETIME', `TIMESTAMP', `YEAR', `SET', `ENUM', and OpenGIS
spatial types. See *Note data-types::.
* Fixed-length and variable-length records.
Statements and Functions:
* Full operator and function support in the `SELECT' and `WHERE'
clauses of queries. For example:
mysql> SELECT CONCAT(first_name, ' ', last_name)
-> FROM citizen
-> WHERE income/dependents > 10000 AND age > 30;
* Full support for SQL `GROUP BY' and `ORDER BY' clauses. Support
for group functions (`COUNT()', `COUNT(DISTINCT ...)', `AVG()',
`STD()', `SUM()', `MAX()', `MIN()', and `GROUP_CONCAT()').
* Support for `LEFT OUTER JOIN' and `RIGHT OUTER JOIN' with both
standard SQL and ODBC syntax.
* Support for aliases on tables and columns as required by standard
SQL.
* `DELETE', `INSERT', `REPLACE', and `UPDATE' return the number of
rows that were changed (affected). It is possible to return the
number of rows matched instead by setting a flag when connecting
to the server.
* The MySQL-specific `SHOW' command can be used to retrieve
information about databases, database engines, tables, and indexes.
The `EXPLAIN' command can be used to determine how the optimizer
resolves a query.
* Function names do not clash with table or column names. For
example, `ABS' is a valid column name. The only restriction is
that for a function call, no spaces are allowed between the
function name and the ‘`('’ that follows it. See *Note
reserved-words::.
* You can mix tables from different databases in the same query (as
of MySQL 3.22).
Security:
* A privilege and password system that is very flexible and secure,
and that allows host-based verification. Passwords are secure
because all password traffic is encrypted when you connect to a
server.
Scalability and Limits:
* Handles large databases. We use MySQL Server with databases that
contain 50 million records. We also know of users who use MySQL
Server with 60,000 tables and about 5,000,000,000 rows.
* Up to 64 indexes per table are allowed (32 before MySQL 4.1.2).
Each index may consist of 1 to 16 columns or parts of columns. The
maximum index width is 1000 bytes (767 for `InnoDB'); before MySQL
4.1.2, the limit is 500 bytes. An index may use a prefix of a
column for `CHAR', `VARCHAR', `BLOB', or `TEXT' column types.
Connectivity:
* Clients can connect to the MySQL server using TCP/IP sockets on
any platform. On Windows systems in the NT family (NT, 2000, XP,
or 2003), clients can connect using named pipes. On Unix systems,
clients can connect using Unix domain socket files.
* In MySQL 4.1 and higher, Windows servers also support
shared-memory connections if started with the -shared-memory
option. Clients can connect through shared memory by using the
-protocol=memory option.
* The Connector/ODBC (MyODBC) interface provides MySQL support for
client programs that use ODBC (Open Database Connectivity)
connections. For example, you can use MS Access to connect to your
MySQL server. Clients can be run on Windows or Unix. MyODBC
source is available. All ODBC 2.5 functions are supported, as are
many others. See *Note connectors::.
* The Connector/J interface provides MySQL support for Java client
programs that use JDBC connections. Clients can be run on Windows
or Unix. Connector/J source is available. See *Note connectors::.
* MySQL Connector/NET enables developers to easily create .NET
applications that require secure, high-performance data
connectivity with MySQL. It implements the required ADO.NET
interfaces and integrates into ADO.NET aware tools. Developers can
build applications using their choice of .NET languages. MySQL
Connector/NET is a fully managed ADO.NET driver written in 100%
pure C#. See *Note connectors::.
Localization:
* The server can provide error messages to clients in many
languages. See *Note languages::.
* Full support for several different character sets, including
`latin1' (cp1252), `german', `big5', `ujis', and more. For
example, the Scandinavian characters ‘`aa'’, ‘`a"'’ and
‘`o"'’ are allowed in table and column names. Unicode support
is available as of MySQL 4.1.
* All data is saved in the chosen character set. All comparisons for
normal string columns are case-insensitive.
* Sorting is done according to the chosen character set (using
Swedish collation by default). It is possible to change this when
the MySQL server is started. To see an example of very advanced
sorting, look at the Czech sorting code. MySQL Server supports
many different character sets that can be specified at compile
time and runtime.
Clients and Tools:
* MySQL Server has built-in support for SQL statements to check,
optimize, and repair tables. These statements are available from
the command line through the `mysqlcheck' client. MySQL also
includes `myisamchk', a very fast command-line utility for
performing these operations on `MyISAM' tables. See *Note
database-administration::.
* All MySQL programs can be invoked with the -help or -? options to
obtain online assistance.
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1.4.3 MySQL Stability
---------------------
This section addresses the questions, `_How stable is MySQL Server?_'
and, `_Can I depend on MySQL Server in this project?_' We will try to
clarify these issues and answer some important questions that concern
many potential users. The information in this section is based on data
gathered from the mailing lists, which are very active in identifying
problems as well as reporting types of use.
The original code stems back to the early 1980s. It provides a stable
code base, and the `ISAM' table format used by the original storage
engine remains backward-compatible. At TcX, the predecessor of MySQL
AB, MySQL code has worked in projects since mid-1996, without any
problems. When the MySQL Database Software initially was released to a
wider public, our new users quickly found some pieces of untested code.
Each new release since then has had fewer portability problems, even
though each new release has also had many new features.
Each release of the MySQL Server has been usable. Problems have
occurred only when users try code from the `gray zones.' Naturally, new
users don't know what the gray zones are; this section therefore
attempts to document those areas that are currently known. The
descriptions mostly deal with Versions 3.23 and later of MySQL Server.
All known and reported bugs are fixed in the latest version, with the
exception of those listed in the bugs section, which are
design-related. See *Note bugs::.
The MySQL Server design is multi-layered with independent modules.
Some of the newer modules are listed here with an indication of how
well-tested each of them is:
* Replication (Stable)
Large groups of servers using replication are in production use,
with good results. Work on enhanced replication features is
continuing.
* `InnoDB' tables (Stable)
The `InnoDB' transactional storage engine has been stable since
version 3.23.49. `InnoDB' is being used in large, heavy-load
production systems.
* `BDB' tables (Stable)
The `Berkeley DB' code is very stable, but we are still improving
the `BDB' transactional storage engine interface in MySQL Server.
* Full-text searches (Stable)
Full-text searching is widely used. Important feature enhancements
were added in MySQL 4.0 and 4.1.
* `MyODBC' 3.51 (Stable)
`MyODBC' 3.51 uses ODBC SDK 3.51 and is in wide production use.
Some issues brought up appear to be application-related and
independent of the ODBC driver or underlying database server.
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1.4.4 How Large MySQL Tables Can Be
-----------------------------------
MySQL 3.22 had a 4GB (4 gigabyte) limit on table size. With the
`MyISAM' storage engine in MySQL 3.23, the maximum table size was
increased to 65536 terabytes (2567 - 1 bytes). With this larger allowed
table size, the maximum effective table size for MySQL databases is
usually determined by operating system constraints on file sizes, not
by MySQL internal limits.
The `InnoDB' storage engine maintains `InnoDB' tables within a
tablespace that can be created from several files. This allows a table
to exceed the maximum individual file size. The tablespace can include
raw disk partitions, which allows extremely large tables. The maximum
tablespace size is 64TB.
The following table lists some examples of operating system file-size
limits. This is only a rough guide and is not intended to be
definitive. For the most up-to-date information, be sure to check the
documentation specific to your operating system.
*Operating System* *File-size Limit*
Linux 2.2-Intel 32-bit 2GB (LFS: 4GB)
Linux 2.4+ (using ext3 filesystem) 4TB
Solaris 9/10 16TB
NetWare w/NSS 8TB
filesystem
Win32 w/ FAT/FAT32 2GB/4GB
Win32 w/ NTFS 2TB (possibly larger)
MacOS X w/ HFS+ 2TB
On Linux 2.2, you can get `MyISAM' tables larger than 2GB in size by
using the Large File Support (LFS) patch for the ext2 filesystem. On
Linux 2.4, patches also exist for ReiserFS to get support for big files
(up to 2TB). Most current Linux distributions are based on kernel 2.4
or higher and include all the required LFS patches. With JFS and XFS,
petabyte and larger files are possible on Linux. However, the maximum
available file size still depends on several factors, one of them being
the filesystem used to store MySQL tables.
For a detailed overview about LFS in Linux, have a look at Andreas
Jaeger's `Large File Support in Linux' page at
`http://www.suse.de/~aj/linux_lfs.html'.
Windows users please note: FAT and VFAT (FAT32) are _not_ considered
suitable for production use with MySQL. Use NTFS instead.
By default, MySQL creates `MyISAM' tables with an internal structure
that allows a maximum size of about 4GB. You can check the maximum
table size for a `MyISAM' table with the `SHOW TABLE STATUS' statement
or with `myisamchk -dv TBL_NAME'. See *Note show::.
If you need a `MyISAM' table that is larger than 4GB and your operating
system supports large files, the `CREATE TABLE' statement supports
`AVG_ROW_LENGTH' and `MAX_ROWS' options. See *Note create-table::. You
can also change these options with `ALTER TABLE' to increase a table's
maximum allowable size after the table has been created. See *Note
alter-table::.
Other ways to work around file-size limits for `MyISAM' tables are as
follows:
* If your large table is read-only, you can use `myisampack' to
compress it. `myisampack' usually compresses a table by at least
50%, so you can have, in effect, much bigger tables. `myisampack'
also can merge multiple tables into a single table. See *Note
myisampack::.
* MySQL includes a `MERGE' library that allows you to handle a
collection of `MyISAM' tables that have identical structure as a
single `MERGE' table. See *Note merge-storage-engine::.
File: manual.info, Node: year-2000-compliance, Prev: table-size, Up: what-is
1.4.5 Year 2000 Compliance
--------------------------
The MySQL Server itself has no problems with Year 2000 (Y2K) compliance:
* MySQL Server uses Unix time functions that handle dates into the
year `2037' for `TIMESTAMP' values. For `DATE' and `DATETIME'
values, dates through the year `9999' are accepted.
* All MySQL date functions are implemented in one source file,
`sql/time.cc', and are coded very carefully to be year 2000-safe.
* In MySQL, the `YEAR' data type can store the years `0' and `1901'
to `2155' in one byte and display them using two or four digits.
All two-digit years are considered to be in the range `1970' to
`2069', which means that if you store `01' in a `YEAR' column,
MySQL Server treats it as `2001'.
The following simple demonstration illustrates that MySQL Server has no
problems with `DATE' or `DATETIME' values through the year 9999, and no
problems with `TIMESTAMP' values until after the year 2030:
mysql> DROP TABLE IF EXISTS y2k;
Query OK, 0 rows affected (0.00 sec)
mysql> CREATE TABLE y2k (date DATE,
-> date_time DATETIME,
-> time_stamp TIMESTAMP);
Query OK, 0 rows affected (0.01 sec)
mysql> INSERT INTO y2k VALUES
-> ('1998-12-31','1998-12-31 23:59:59','1998-12-31 23:59:59'),
-> ('1999-01-01','1999-01-01 00:00:00','1999-01-01 00:00:00'),
-> ('1999-09-09','1999-09-09 23:59:59','1999-09-09 23:59:59'),
-> ('2000-01-01','2000-01-01 00:00:00','2000-01-01 00:00:00'),
-> ('2000-02-28','2000-02-28 00:00:00','2000-02-28 00:00:00'),
-> ('2000-02-29','2000-02-29 00:00:00','2000-02-29 00:00:00'),
-> ('2000-03-01','2000-03-01 00:00:00','2000-03-01 00:00:00'),
-> ('2000-12-31','2000-12-31 23:59:59','2000-12-31 23:59:59'),
-> ('2001-01-01','2001-01-01 00:00:00','2001-01-01 00:00:00'),
-> ('2004-12-31','2004-12-31 23:59:59','2004-12-31 23:59:59'),
-> ('2005-01-01','2005-01-01 00:00:00','2005-01-01 00:00:00'),
-> ('2030-01-01','2030-01-01 00:00:00','2030-01-01 00:00:00'),
-> ('2040-01-01','2040-01-01 00:00:00','2040-01-01 00:00:00'),
-> ('9999-12-31','9999-12-31 23:59:59','9999-12-31 23:59:59');
Query OK, 14 rows affected, 2 warnings (0.00 sec)
Records: 14 Duplicates: 0 Warnings: 2
mysql> SELECT * FROM y2k;
+------------+---------------------+---------------------+
| date | date_time | time_stamp |
+------------+---------------------+---------------------+
| 1998-12-31 | 1998-12-31 23:59:59 | 1998-12-31 23:59:59 |
| 1999-01-01 | 1999-01-01 00:00:00 | 1999-01-01 00:00:00 |
| 1999-09-09 | 1999-09-09 23:59:59 | 1999-09-09 23:59:59 |
| 2000-01-01 | 2000-01-01 00:00:00 | 2000-01-01 00:00:00 |
| 2000-02-28 | 2000-02-28 00:00:00 | 2000-02-28 00:00:00 |
| 2000-02-29 | 2000-02-29 00:00:00 | 2000-02-29 00:00:00 |
| 2000-03-01 | 2000-03-01 00:00:00 | 2000-03-01 00:00:00 |
| 2000-12-31 | 2000-12-31 23:59:59 | 2000-12-31 23:59:59 |
| 2001-01-01 | 2001-01-01 00:00:00 | 2001-01-01 00:00:00 |
| 2004-12-31 | 2004-12-31 23:59:59 | 2004-12-31 23:59:59 |
| 2005-01-01 | 2005-01-01 00:00:00 | 2005-01-01 00:00:00 |
| 2030-01-01 | 2030-01-01 00:00:00 | 2030-01-01 00:00:00 |
| 2040-01-01 | 2040-01-01 00:00:00 | 0000-00-00 00:00:00 |
| 9999-12-31 | 9999-12-31 23:59:59 | 0000-00-00 00:00:00 |
+------------+---------------------+---------------------+
14 rows in set (0.00 sec)
The final two `TIMESTAMP' column values are zero because the year
values (`2040', `9999') exceed the `TIMESTAMP' maximum. The `TIMESTAMP'
data type, which is used to store the current time, supports values
that range from `'1970-01-01 00:00:00'' to `'2030-01-01 00:00:00'' on
32-bit machines (signed value). On 64-bit machines, `TIMESTAMP' handles
values up to `2106' (unsigned value).
Although MySQL Server itself is Y2K-safe, you may run into problems if
you use it with applications that are not Y2K-safe. For example, many
old applications store or manipulate years using two-digit values
(which are ambiguous) rather than four-digit values. This problem may
be compounded by applications that use values such as `00' or `99' as
`missing' value indicators. Unfortunately, these problems may be
difficult to fix because different applications may be written by
different programmers, each of whom may use a different set of
conventions and date-handling functions.
Thus, even though MySQL Server has no Y2K problems, _it is the
application's responsibility to provide unambiguous input_. See *Note
y2k-issues::, for MySQL Server's rules for dealing with ambiguous date
input data that contains two-digit year values.
File: manual.info, Node: maxdb, Next: roadmap, Prev: what-is, Up: introduction
1.5 Overview of the MaxDB Database Management System
====================================================
* Menu:
* maxdb-overview:: What is MaxDB?
* maxdb-history:: History of MaxDB
* maxdb-features:: Features of MaxDB
* maxdb-licensing:: Licensing and Support
* maxdb-mysql-differences:: Feature Differences Between MaxDB and MySQL
* maxdb-mysql-interoperability:: Interoperability Features Between MaxDB and MySQL
* maxdb-links:: MaxDB-Related Links
MaxDB is a heavy-duty enterprise database. The database management
system is SAP-certified.
MaxDB is the new name of a database management system formerly called
SAP DB. In 2003 SAP AG and MySQL AB joined a partnership and re-branded
the database system to MaxDB. The development of MaxDB has continued
since then as it was done before--through the SAP developer team.
MySQL AB cooperates closely with the MaxDB team at SAP around
delivering improvements to the MaxDB product. Joint efforts include
development of new native drivers to enable more efficient usage of
MaxDB in the Open Source community, and improvement of documentation to
expand the MaxDB user base. Interoperability features between MySQL and
MaxDB database also are seen as important. For example, the new MaxDB
Synchronization Manager supports data synchronization from MaxDB to
MySQL.
The MaxDB database management system does not share a common code-base
with the MySQL database management system. The MaxDB and MySQL database
management systems are independent products provided by MySQL AB.
MySQL AB offers a complete portfolio of Professional Services for MaxDB.
File: manual.info, Node: maxdb-overview, Next: maxdb-history, Prev: maxdb, Up: maxdb
1.5.1 What is MaxDB?
--------------------
MaxDB is an ANSI SQL-92 (entry level) compliant relational database
management system (RDBMS) from SAP AG, that is delivered by MySQL AB as
well. MaxDB fulfills the needs for enterprise usage: safety,
scalability, high concurrency, and performance. It runs on all major
operating systems. Over the years it has proven able to run SAP R/3 and
terabytes of data in 24×7 operation.
The database development started in 1977 as a research project at the
Technical University of Berlin. In the early 1980s it became a database
product that subsequently was owned by Nixdorf, Siemens Nixdorf,
Software AG, and today by SAP AG. Along the way, it has been named VDN,
Reflex, Supra 2, DDB/4, Entire SQL-DB-Server, and ADABAS D. In 1997,
SAP took over the software from Software AG and renamed it to SAP DB.
Since October 2000, SAP DB sources additionally were released as Open
Source under the GNU General Public License (see *Note gpl-license::).
In 2003, SAP AG and MySQL AB formed a partnership and re-branded the
database system to MaxDB.
File: manual.info, Node: maxdb-history, Next: maxdb-features, Prev: maxdb-overview, Up: maxdb
1.5.2 History of MaxDB
----------------------
The history of MaxDB goes back to SAP DB, SAP AG's DBMS. That is, MaxDB
is a re-branded and enhanced version of SAP DB. For many years, MaxDB
has been used for small, medium, and large installations of the mySAP
Business Suite and other demanding SQL applications requiring an
enterprise-class DBMS with regard to the number of users, the
transactional workload, and the size of the database.
SAP DB was meant to provide an alternative to third-party database
systems such as Oracle, Microsoft SQL Server, and DB2 by IBM. In
October 2000, SAP AG released SAP DB under the GNU GPL license (see
*Note gpl-license::), thus making it Open Source software.
Today, MaxDB is used in about 3,500 SAP customer installations
worldwide. Moreover, the majority of all DBMS installations on Unix and
Linux within SAP’s IT department rely on MaxDB. MaxDB is tuned toward
heavy-duty online transaction processing (OLTP) with several thousand
users and database sizes ranging from several hundred GB to multiple TB.
In 2003, SAP and MySQL concluded a partnership and development
cooperation agreement. As a result, SAP's database system SAP DB has
been delivered under the name of MaxDB by MySQL since the release of
version 7.5 (November 2003).
Version 7.5 of MaxDB is a direct advancement of the SAP DB 7.4 code
base. Therefore, the MaxDB software version 7.5 can be used as a direct
upgrade of previous SAP DB versions starting 7.2.04 and higher.
The former SAP DB development team at SAP AG is responsible, now as
before, for developing and supporting MaxDB. MySQL AB cooperates
closely with the MaxDB team at SAP around delivering improvements to
the MaxDB product, see *Note maxdb::. Both SAP AG and MySQL AB handle
the sale and distribution of MaxDB. The advancement of MaxDB and the
MySQL Server leverages synergies that benefit both product lines.
MaxDB is subjected to SAP AG's complete quality assurance process
before it is shipped with SAP solutions or provided as a download from
the MySQL site.
File: manual.info, Node: maxdb-features, Next: maxdb-licensing, Prev: maxdb-history, Up: maxdb
1.5.3 Features of MaxDB
-----------------------
MaxDB is a heavy-duty, SAP-certified Open Source database for OLTP and
OLAP usage which offers high reliability, availability, scalability,
and a very comprehensive feature set. It is targeted for large mySAP
Business Suite environments and other applications that require maximum
enterprise-level database functionality and complements the MySQL
database server.
MaxDB operates as a client/server product. It was developed to meet the
needs of installations in OLTP and Data Warehouse/OLAP/Decision Support
scenarios and offers these benefits:
* *Easy configuration and administration:* GUI-based Installation
Manager and Database Manager as single administration tools for
DBMS operations
* *Around-the-clock operation, no planned downtimes, no permanent
attendance required:* Automatic space management, no need for
reorganizations
* *Sophisticated backup and restore capabilities:* Online and
incremental backups, recovery wizard to guide you through the
recovery scenario
* *Supports large number of users, database sizes in the terabytes,
and demanding workloads:* Proven reliability, performance, and
scalability
* *High availability:* Cluster support, standby configuration, hot
standby configuration
File: manual.info, Node: maxdb-licensing, Next: maxdb-mysql-differences, Prev: maxdb-features, Up: maxdb
1.5.4 Licensing and Support
---------------------------
MaxDB can be used under the same licenses available for the other
products distributed by MySQL AB. Thus, MaxDB is available under the
GNU General Public License, and a commercial license. For more
information on licensing, see
`http://www.mysql.com/company/legal/licensing/'.
MySQL AB offers MaxDB technical support to non-SAP customers. MaxDB
support is available on various levels (Basic, Silver, and Gold), which
expand from unlimited email/web-support to 24×7 phone support for
business critical systems.
MySQL AB also offers Licenses and Support for MaxDB when used with SAP
Applications, like SAP NetWeaver and mySAP Business Suite. For more
information on licenses and support for your needs, please contact
MySQL AB. (See `http://www.mysql.com/company/contact/'.)
Consulting and training services are available. MySQL gives classes on
MaxDB at regular intervals. See `http://www.mysql.com/training/' for a
list of classes.
File: manual.info, Node: maxdb-mysql-differences, Next: maxdb-mysql-interoperability, Prev: maxdb-licensing, Up: maxdb
1.5.5 Feature Differences Between MaxDB and MySQL
-------------------------------------------------
MaxDB is MySQL AB's SAP-certified database. The MaxDB database server
complements the MySQL AB product portfolio. Some MaxDB features are not
available on the MySQL database management server and vice versa.
The following list summarizes the main differences between MaxDB and
MySQL; it is not complete.
* MaxDB runs as a client/server system. MySQL can run as a
client/server system or as an embedded system.
* MaxDB might not run on all platforms supported by MySQL.
* MaxDB uses a proprietary network protocol for client/server
communication. MySQL uses either TCP/IP (with or without SSL
encryption), sockets (under Unix-like systems), or named pipes or
shared memory (under Windows NT-family systems).
* MaxDB supports stored procedures and functions. MySQL 5.0 and up
also supports stored procedures and function and functions. MaxDB
supports programming of triggers through an SQL extension. MySQL
5.0 supports triggers. MaxDB contains a debugger for stored
procedure languages, can cascade nested triggers, and supports
multiple triggers per action and row.
* MaxDB is distributed with user interfaces that are text-based,
graphical, or Web-based. MySQL is distributed with text-based user
interfaces only; graphical user interfaces (MySQL Query Browser,
MySQL Administrator) are shipped separately from the main
distributions. Web-based user interfaces for MySQL are offered by
third parties.
* MaxDB supports a number of programming interfaces that also are
supported by MySQL. For developing with MaxDB, the MaxDB ODBC
Driver, SQL Database Connectivity (SQLDBC), JDBC Driver, Perl and
Python modules and a MaxDB PHP extension, which provides access to
MySQL MaxDB databases using PHP, are available. Third Party
Programming Interfaces: Support for OLE DB, ADO, DAO, RDO and .NET
through ODBC. MaxDB supports embedded SQL with C/C++.
* MaxDB includes administrative features that MySQL does not have:
job scheduling by time, event, and alert, and sending messages to
a database administrator on alert thresholds. (MySQL has
scheduling support starting with version 5.1.6.)
File: manual.info, Node: maxdb-mysql-interoperability, Next: maxdb-links, Prev: maxdb-mysql-differences, Up: maxdb
1.5.6 Interoperability Features Between MaxDB and MySQL
-------------------------------------------------------
MaxDB and MySQL are independent database management servers. The
interoperation of the systems is possible in a way that the systems can
exchange their data. To exchange data between MaxDB and MySQL, you can
use the import and export tools of the systems or the MaxDB
Synchronization Manager. The import and export tools can be used to
transfer data in an infrequent, manual fashion. The MaxDB
Synchronization Manager offers faster, automatic data transfer
capabilities.
The MaxDB Loader can be used to export data and object definitions. The
Loader can export data using MaxDB internal, binary formats and text
formats (CSV). Data exported from MaxDB in text formats can be imported
into MySQL using the `mysqlimport' client program. To export MySQL
data, you can use either `mysqldump' to create `INSERT' statements or
`SELECT ... INTO OUTFILE' to create a text file (CSV). Use the MaxDB
Loader to import the data files generated by MySQL.
Object definitions can be exchanged between the systems using MaxDB
Loader and the MySQL tool `mysqldump'. As the SQL dialects of both
systems differ slightly and MaxDB has features currently not supported
by MySQL like SQL constraints, we recommend to hand-tune the definition
files. The `mysqldump' tool offers an option -compatible=maxdb to
produce output that is compatible with MaxDB to make porting easier.
The MaxDB Synchronization Manager is available as part of MaxDB 7.6.
The Synchronization Manager supports creation of asynchronous
replication scenarios between several MaxDB instances. However,
interoperability features also are planned, so that the Synchronization
Manager supports replication to and from a MySQL server.
In the first release, the Synchronization Manager supports inserting
data into MySQL. This means that initially only replication from MaxDB
to MySQL is supported. In the course of 2005, exporting of data from a
MySQL server to the Synchronization Manager will be added, thus adding
support for MySQL to MaxDB replication scenarios.
File: manual.info, Node: maxdb-links, Prev: maxdb-mysql-interoperability, Up: maxdb
1.5.7 MaxDB-Related Links
-------------------------
The main page for MaxDB information is
`http://www.mysql.com/products/maxdb', which provides details about the
features of the MaxDB database management systems and has pointers to
available documentation.
The MySQL Reference Manual does not contain any MaxDB documentation
other than the introduction given in this section. MaxDB has its own
documentation, which is called the MaxDB library and is available at
`http://dev.mysql.com/doc/maxdb/index.html'.
MySQL AB runs a community mailing list on MaxDB; see
`http://lists.mysql.com/maxdb'. The list shows a vivid community
discussion. Many of the core developers contribute to it. Product
announcements are sent to the list.
A Web forum on MaxDB is available at `http://forums.mysql.com/'. The
forum focuses on MaxDB questions not related to SAP applications.
File: manual.info, Node: roadmap, Next: information-sources, Prev: maxdb, Up: introduction
1.6 MySQL Development Roadmap
=============================
* Menu:
* mysql-5-0-nutshell:: What's New in MySQL 5.0
This section provides a snapshot of the MySQL development roadmap,
including major features implemented in or planned for various MySQL
releases. The following sections provide information for each release
series.
The current production release series is MySQL 5.0, which was declared
stable for production use as of MySQL 5.0.15, released in October 2005.
The previous production release series was MySQL 4.1, which was
declared stable for production use as of MySQL 4.1.7, released in
October 2004. `Production status' means that future 5.0 and 4.1
development is limited only to bugfixes. For the older MySQL 4.0 and
3.23 series, only critical bugfixes are made.
Active MySQL development is currently taking place in the MySQL 5.0 and
5.1 release series, and new features are being added only to the latter.
Before upgrading from one release series to the next, please see the
notes in *Note upgrade::.
The most requested features and the versions in which they were
implemented or are scheduled for implementation are summarized in the
following table:
*Feature* *MySQL Series*
Foreign keys 3.23 (for the `InnoDB' storage engine)
Unions 4.0
Subqueries 4.1
R-trees 4.1 (for the `MyISAM' storage engine)
Stored procedures 5.0
Views 5.0
Cursors 5.0
XA transactions 5.0
Foreign keys 5.2 (implemented in 3.23 for `InnoDB')
Triggers 5.0 and 5.1
Partitioning 5.1
Pluggable Storage 5.1
Engine API
Row-Based Replication 5.1
File: manual.info, Node: mysql-5-0-nutshell, Prev: roadmap, Up: roadmap
1.6.1 What's New in MySQL 5.0
-----------------------------
The following features are implemented in MySQL 5.0.
* *`BIT' Data Type*: Can be used to store numbers in binary
notation. See *Note numeric-type-overview::.
* *Cursors*: Elementary support for server-side cursors. For
information about using cursors within stored routines, see *Note
cursors::. For information about using cursors from within the C
API, see *Note mysql-stmt-attr-set::.
* *Information Schema*: The introduction of the `INFORMATION_SCHEMA'
database in MySQL 5.0 provided a standards-compliant means for
accessing the MySQL Server's metadata; that is, data about the
databases (schemas) on the server and the objects which they
contain. See *Note information-schema::.
* *Instance Manager*: Can be used to start and stop the MySQL
Server, even from a remote host. See *Note instance-manager::.
* *Precision Math*: MySQL 5.0 introduced stricter criteria for
acceptance or rejection of data, and implemented a new library for
fixed-point arithmetic. These contributed to a much higher degree
of accuracy for mathematical operations and greater control over
invalid values. See *Note precision-math::.
* *Storage Engines*: Storage engines added in MySQL 5.0 include
`ARCHIVE' and `FEDERATED'. See *Note archive-storage-engine::, and
*Note federated-storage-engine::.
* *Stored Routines*: Support for named stored procedures and stored
functions was implemented in MySQL 5.0. See *Note
stored-procedures::.
* *Strict Mode and Standard Error Handling*: MySQL 5.0 added a
strict mode where by it follows standard SQL in a number of ways
in which it did not previously. Support for standard SQLSTATE
error messages was also implemented. See *Note server-sql-mode::.
* *Triggers*: MySQL 5.0 added limited support for triggers. See
*Note triggers::, and *Note ansi-diff-triggers::.
* *`VARCHAR' Data Type*: The maximum effective length of a `VARCHAR'
column was increased to 65,532 bytes, and stripping of trailing
whitespace was eliminated. See *Note string-types::.
* *Views*: MySQL 5.0 added support for named, updatable views. See
*Note views::, and *Note ansi-diff-views::.
* *XA Transactions*: See *Note xa::.
* *Performance enhancements*: A number of improvements were made in
MySQL 5.0 to improve the speed of certain types of queries and in
the handling of certain types. These include:
* MySQL 5.0 introduces a new `greedy' optimizer which can
greatly reduce the time required to arrive at a query
execution plan. This is particularly noticeable where several
tables are to be joined and no good join keys can otherwise
be found. Without the greedy optimizer, the complexity of the
search for an execution plan is calculated as `N!', where N
is the number of tables to be joined. The greedy optimizer
reduces this to `N!/(D-1)!', where D is the depth of the
search. Although the greedy optimizer does not guarantee the
best possible of all execution plans (this is currently being
worked on), it can reduce the time spent arriving at an
execution plan for a join involving a great many tables --
30, 40, or more -- by a factor of as much as 1,000. This
should eliminate most if not all situations where users
thought that the optimizer had hung when trying to perform
joins across many tables.
* Use of the Index Merge method to obtain better optimization
of `AND' and `OR' relations over different keys.
(Previously, these were optimized only where both relations
in the `WHERE' clause involved the same key.) This also
applies to other one-to-one comparison operators (`>', `<',
and so on), including `=' and the `IN' operator. This means
that MySQL can use multiple indexes in retrieving results for
conditions such as `WHERE key1 > 4 OR key2 < 7' and even
combinations of conditions such as `WHERE (key1 > 4 OR key2 <
7) AND (key3 >= 10 OR key4 = 1)'. See *Note
index-merge-optimization::.
* A new equality detector finds and optimizes `hidden'
equalities in joins. For example, a `WHERE' clause such as
t1.c1=t2.c2 AND t2.c2=t3.c3 AND t1.c1 < 5
can be reduced to
t1.c1=t3.c3 AND t2.c2 < 5 AND t3.c3 < 5
These optimizations can be applied with any combination of
`AND' and `OR' operators. See *Note nested-joins::, and *Note
outer-join-simplification::.
* Optimization of `NOT IN' and `NOT BETWEEN' relations,
reducing or eliminating table scans for queries making use of
them by mean of range analysis. The performance of MySQL with
regard to these relations now matches its performance with
regard to `IN' and `BETWEEN'.
* The `VARCHAR' data type as implemented in MySQL 5.0 is more
efficient than in previous versions, due to the elimination
of the old (and nonstandard) removal of trailing spaces during
retrieval.
* The addition of a true `BIT' column type; this type is much
more efficient for storage and retrieval of Boolean values
than the workarounds required in MySQL in versions previous
to 5.0.
* *Performance Improvements in the `InnoDB' Storage Engine*:
* New compact storage format which can save up to 20% of
the disk space required in previous MySQL/`InnoDB'
versions.
* Faster recovery from a failed or aborted `ALTER TABLE'.
* Faster implementation of `TRUNCATE'.
(See *Note innodb::.)
* *Performance Improvements in the `NDBCluster' Storage Engine*:
* Faster handling of queries that use `IN' and `BETWEEN'.
* *Condition pushdown*: In cases involving the comparison
of an unindexed column with a constant, this condition is
`pushed down' to the cluster where it is evaluated in
all partitions simultaneously, eliminating the need to
send non-matching records over the network. This can
make such queries 10 to 100 times faster than in MySQL
4.1 Cluster.
See *Note explain::, for more information.
(See *Note ndbcluster::.)
For those wishing to take a look at the bleeding edge of MySQL
development, we make our BitKeeper repository for MySQL publicly
available. See *Note installing-source-tree::.
File: manual.info, Node: information-sources, Next: bug-reports, Prev: roadmap, Up: introduction
1.7 MySQL Information Sources
=============================
* Menu:
* mailing-lists:: MySQL Mailing Lists
* forums:: MySQL Community Support at the MySQL Forums
* irc:: MySQL Community Support on Internet Relay Chat (IRC)
This section lists sources of additional information that you may find
helpful, such as the MySQL mailing lists and user forums, and Internet
Relay Chat.
File: manual.info, Node: mailing-lists, Next: forums, Prev: information-sources, Up: information-sources
1.7.1 MySQL Mailing Lists
-------------------------
* Menu:
* mailing-list-use:: Guidelines for Using the Mailing Lists
This section introduces the MySQL mailing lists and provides guidelines
as to how the lists should be used. When you subscribe to a mailing
list, you receive all postings to the list as email messages. You can
also send your own questions and answers to the list.
To subscribe to or unsubscribe from any of the mailing lists described
in this section, visit `http://lists.mysql.com/'. For most of them, you
can select the regular version of the list where you get individual
messages, or a digest version where you get one large message per day.
Please _do not_ send messages about subscribing or unsubscribing to any
of the mailing lists, because such messages are distributed
automatically to thousands of other users.
Your local site may have many subscribers to a MySQL mailing list. If
so, the site may have a local mailing list, so that messages sent from
`lists.mysql.com' to your site are propagated to the local list. In
such cases, please contact your system administrator to be added to or
dropped from the local MySQL list.
If you wish to have traffic for a mailing list go to a separate mailbox
in your mail program, set up a filter based on the message headers. You
can use either the `List-ID:' or `Delivered-To:' headers to identify
list messages.
The MySQL mailing lists are as follows:
* `announce'
This list is for announcements of new versions of MySQL and
related programs. This is a low-volume list to which all MySQL
users should subscribe.
* `mysql'
This is the main list for general MySQL discussion. Please note
that some topics are better discussed on the more-specialized
lists. If you post to the wrong list, you may not get an answer.
* `bugs'
This list is for people who want to stay informed about issues
reported since the last release of MySQL or who want to be
actively involved in the process of bug hunting and fixing. See
*Note bug-reports::.
* `internals'
This list is for people who work on the MySQL code. This is also
the forum for discussions on MySQL development and for posting
patches.
* `mysqldoc'
This list is for people who work on the MySQL documentation:
people from MySQL AB, translators, and other community members.
* `benchmarks'
This list is for anyone interested in performance issues.
Discussions concentrate on database performance (not limited to
MySQL), but also include broader categories such as performance of
the kernel, filesystem, disk system, and so on.
* `packagers'
This list is for discussions on packaging and distributing MySQL.
This is the forum used by distribution maintainers to exchange
ideas on packaging MySQL and on ensuring that MySQL looks and
feels as similar as possible on all supported platforms and
operating systems.
* `java'
This list is for discussions about the MySQL server and Java. It
is mostly used to discuss JDBC drivers such as MySQL Connector/J.
* `win32'
This list is for all topics concerning the MySQL software on
Microsoft operating systems, such as Windows 9x, Me, NT, 2000, XP,
and 2003.
* `myodbc'
This list is for all topics concerning connecting to the MySQL
server with ODBC.
* `gui-tools'
This list is for all topics concerning MySQL graphical user
interface tools such as `MySQL Administrator' and `MySQL Query
Browser'.
* `cluster'
This list is for discussion of MySQL Cluster.
* `dotnet'
This list is for discussion of the MySQL server and the .NET
platform. It is mostly related to MySQL Connector/Net.
* `plusplus'
This list is for all topics concerning programming with the C++
API for MySQL.
* `perl'
This list is for all topics concerning Perl support for MySQL with
`DBD::mysql'.
If you're unable to get an answer to your questions from a MySQL
mailing list or forum, one option is to purchase support from MySQL AB.
This puts you in direct contact with MySQL developers.
The following table shows some MySQL mailing lists in languages other
than English. These lists are not operated by MySQL AB.
* `<mysql-france-subscribe AT yahoogroups.com>'
A French mailing list.
* `<list AT tinc.net>'
A Korean mailing list. To subscribe, email `subscribe mysql
your AT email.address' to this list.
* `<mysql-de-request AT lists.com>'
A German mailing list. To subscribe, email `subscribe mysql-de
your AT email.address' to this list. You can find information about
this mailing list at `http://www.4t2.com/mysql/'.
* `<mysql-br-request AT listas.br>'
A Portuguese mailing list. To subscribe, email `subscribe mysql-br
your AT email.address' to this list.
* `<mysql-alta AT elistas.net>'
A Spanish mailing list. To subscribe, email `subscribe mysql
your AT email.address' to this list.
File: manual.info, Node: mailing-list-use, Prev: mailing-lists, Up: mailing-lists
1.7.1.1 Guidelines for Using the Mailing Lists
..............................................
Please don't post mail messages from your browser with HTML mode turned
on. Many users don't read mail with a browser.
When you answer a question sent to a mailing list, if you consider your
answer to have broad interest, you may want to post it to the list
instead of replying directly to the individual who asked. Try to make
your answer general enough that people other than the original poster
may benefit from it. When you post to the list, please make sure that
your answer is not a duplication of a previous answer.
Try to summarize the essential part of the question in your reply.
Don't feel obliged to quote the entire original message.
When answers are sent to you individually and not to the mailing list,
it is considered good etiquette to summarize the answers and send the
summary to the mailing list so that others may have the benefit of
responses you received that helped you solve your problem.
File: manual.info, Node: forums, Next: irc, Prev: mailing-lists, Up: information-sources
1.7.2 MySQL Community Support at the MySQL Forums
-------------------------------------------------
The forums at `http://forums.mysql.com' are an important community
resource. Many forums are available, grouped into these general
categories:
* Migration
* MySQL Usage
* MySQL Connectors
* Programming Languages
* Tools
* 3rd-Party Applications
* Storage Engines
* MySQL Technology
* SQL Standards
* Business
File: manual.info, Node: irc, Prev: forums, Up: information-sources
1.7.3 MySQL Community Support on Internet Relay Chat (IRC)
----------------------------------------------------------
In addition to the various MySQL mailing lists and forums, you can find
experienced community people on Internet Relay Chat (IRC). These are
the best networks/channels currently known to us:
*freenode* (see `http://www.freenode.net/' for servers)
* `#mysql' is primarily for MySQL questions, but other database and
general SQL questions are welcome. Questions about PHP, Perl, or
C in combination with MySQL are also common.
If you are looking for IRC client software to connect to an IRC
network, take a look at `xChat' (`http://www.xchat.org/'). X-Chat (GPL
licensed) is available for Unix as well as for Windows platforms (a free
Windows build of X-Chat is available at
`http://www.silverex.org/download/').
File: manual.info, Node: bug-reports, Next: compatibility, Prev: information-sources, Up: introduction
1.8 How to Report Bugs or Problems
==================================
Before posting a bug report about a problem, please try to verify that
it is a bug and that it has not been reported already:
* Start by searching the MySQL online manual at
`http://dev.mysql.com/doc/'. We try to keep the manual up to date
by updating it frequently with solutions to newly found problems.
The change history (`http://dev.mysql.com/doc/mysql/en/news.html')
can be particularly useful since it is quite possible that a newer
version contains a solution to your problem.
* If you get a parse error for a SQL statement, please check your
syntax closely. If you can't find something wrong with it, it's
extremely likely that your current version of MySQL Server doesn't
support the syntax you are using. If you are using the current
version and the manual doesn't cover the syntax that you are
using, MySQL Server doesn't support your statement. In this case,
your options are to implement the syntax yourself or email
<licensing AT mysql.com> and ask for an offer to implement it.
If the manual covers the syntax you are using, but you have an
older version of MySQL Server, you should check the MySQL change
history to see when the syntax was implemented. In this case, you
have the option of upgrading to a newer version of MySQL Server.
* For solutions to some common problems, see *Note problems::.
* Search the bugs database at `http://bugs.mysql.com/' to see
whether the bug has been reported and fixed.
* Search the MySQL mailing list archives at
`http://lists.mysql.com/'. See *Note mailing-lists::.
* You can also use `http://www.mysql.com/search/' to search all the
Web pages (including the manual) that are located at the MySQL AB
Web site.
If you can't find an answer in the manual, the bugs database, or the
mailing list archives, check with your local MySQL expert. If you still
can't find an answer to your question, please use the following
guidelines for reporting the bug.
The normal way to report bugs is to visit `http://bugs.mysql.com/',
which is the address for our bugs database. This database is public and
can be browsed and searched by anyone. If you log in to the system, you
can enter new reports. If you have no Web access, you can generate a
bug report by using the `mysqlbug' script described at the end of this
section.
Bugs posted in the bugs database at `http://bugs.mysql.com/' that are
corrected for a given release are noted in the change history.
If you have found a sensitive security bug in MySQL, you can send email
to <security AT mysql.com>.
To discuss problems with other users, you can use one of the MySQL
mailing lists. *Note mailing-lists::.
Writing a good bug report takes patience, but doing it right the first
time saves time both for us and for yourself. A good bug report,
containing a full test case for the bug, makes it very likely that we
will fix the bug in the next release. This section helps you write your
report correctly so that you don't waste your time doing things that
may not help us much or at all. Please read this section carefully and
make sure that all the information described here is included in your
report.
Preferably, you should test the problem using the latest production or
development version of MySQL Server before posting. Anyone should be
able to repeat the bug by just using `mysql test < script_file' on your
test case or by running the shell or Perl script that you include in
the bug report. Any bug that we are able to repeat has a high chance of
being fixed in the next MySQL release.
It is most helpful when a good description of the problem is included
in the bug report. That is, give a good example of everything you did
that led to the problem and describe, in exact detail, the problem
itself. The best reports are those that include a full example showing
how to reproduce the bug or problem. See *Note reproducible-test-case::.
Remember that it is possible for us to respond to a report containing
too much information, but not to one containing too little. People
often omit facts because they think they know the cause of a problem
and assume that some details don't matter. A good principle to follow
is that if you are in doubt about stating something, state it. It is
faster and less troublesome to write a couple more lines in your report
than to wait longer for the answer if we must ask you to provide
information that was missing from the initial report.
The most common errors made in bug reports are (a) not including the
version number of the MySQL distribution that you use, and (b) not
fully describing the platform on which the MySQL server is installed
(including the platform type and version number). These are highly
relevant pieces of information, and in 99 cases out of 100, the bug
report is useless without them. Very often we get questions like, `Why
doesn't this work for me?' Then we find that the feature requested
wasn't implemented in that MySQL version, or that a bug described in a
report has been fixed in newer MySQL versions. Errors often are
platform-dependent. In such cases, it is next to impossible for us to
fix anything without knowing the operating system and the version
number of the platform.
If you compiled MySQL from source, remember also to provide information
about your compiler if it is related to the problem. Often people find
bugs in compilers and think the problem is MySQL-related. Most
compilers are under development all the time and become better version
by version. To determine whether your problem depends on your compiler,
we need to know what compiler you used. Note that every compiling
problem should be regarded as a bug and reported accordingly.
If a program produces an error message, it is very important to include
the message in your report. If we try to search for something from the
archives, it is better that the error message reported exactly matches
the one that the program produces. (Even the lettercase should be
observed.) It is best to copy and paste the entire error message into
your report. You should never try to reproduce the message from memory.
If you have a problem with Connector/ODBC (MyODBC), please try to
generate a trace file and send it with your report. See *Note
myodbc-bug-report::.
If your report includes long query output lines from test cases that
you run with the `mysql' command-line tool, you can make the output
more readable by using the -vertical option or the `\G' statement
terminator. The `EXPLAIN SELECT' example later in this section
demonstrates the use of `\G'.
Please include the following information in your report:
* The version number of the MySQL distribution you are using (for
example, MySQL 5.0.19). You can find out which version you are
running by executing `mysqladmin version'. The `mysqladmin'
program can be found in the `bin' directory under your MySQL
installation directory.
* The manufacturer and model of the machine on which you experience
the problem.
* The operating system name and version. If you work with Windows,
you can usually get the name and version number by double-clicking
your My Computer icon and pulling down the `Help/About Windows'
menu. For most Unix-like operating systems, you can get this
information by executing the command `uname -a'.
* Sometimes the amount of memory (real and virtual) is relevant. If
in doubt, include these values.
* If you are using a source distribution of the MySQL software,
include the name and version number of the compiler that you used.
If you have a binary distribution, include the distribution name.
* If the problem occurs during compilation, include the exact error
messages and also a few lines of context around the offending code
in the file where the error occurs.
* If `mysqld' died, you should also report the statement that
crashed `mysqld'. You can usually get this information by running
`mysqld' with query logging enabled, and then looking in the log
after `mysqld' crashes. See *Note using-log-files::.
* If a database table is related to the problem, include the output
from the `SHOW CREATE TABLE DB_NAME.TBL_NAME' statement in the bug
report. This is a very easy way to get the definition of any table
in a database. The information helps us create a situation
matching the one that you have experienced.
* For performance-related bugs or problems with `SELECT' statements,
you should always include the output of `EXPLAIN SELECT ...', and
at least the number of rows that the `SELECT' statement produces.
You should also include the output from `SHOW CREATE TABLE
TBL_NAME' for each table that is involved. The more information
you provide about your situation, the more likely it is that
someone can help you.
The following is an example of a very good bug report. The
statements are run using the `mysql' command-line tool. Note the
use of the `\G' statement terminator for statements that would
otherwise provide very long output lines that are difficult to
read.
mysql> SHOW VARIABLES;
mysql> SHOW COLUMNS FROM ...\G
<OUTPUT FROM SHOW COLUMNS>
mysql> EXPLAIN SELECT ...\G
<OUTPUT FROM EXPLAIN>
mysql> FLUSH STATUS;
mysql> SELECT ...;
<A SHORT VERSION OF THE OUTPUT FROM SELECT,
INCLUDING THE TIME TAKEN TO RUN THE QUERY>
mysql> SHOW STATUS;
<OUTPUT FROM SHOW STATUS>
* If a bug or problem occurs while running `mysqld', try to provide
an input script that reproduces the anomaly. This script should
include any necessary source files. The more closely the script
can reproduce your situation, the better. If you can make a
reproducible test case, you should upload it to be attached to the
bug report.
If you can't provide a script, you should at least include the
output from `mysqladmin variables extended-status processlist' in
your report to provide some information on how your system is
performing.
* If you can't produce a test case with only a few rows, or if the
test table is too big to be included in the bug report (more than
10 rows), you should dump your tables using `mysqldump' and create
a `README' file that describes your problem. Create a compressed
archive of your files using `tar' and `gzip' or `zip', and use FTP
to transfer the archive to
`ftp://ftp.mysql.com/pub/mysql/upload/'. Then enter the problem
into our bugs database at `http://bugs.mysql.com/'.
* If you believe that the MySQL server produces a strange result
from a statement, include not only the result, but also your
opinion of what the result should be, and an explanation
describing the basis for your opinion.
* When you provide an example of the problem, it's better to use the
table names, variable names, and so forth that exist in your
actual situation than to come up with new names. The problem could
be related to the name of a table or variable. These cases are
rare, perhaps, but it is better to be safe than sorry. After all,
it should be easier for you to provide an example that uses your
actual situation, and it is by all means better for us. If you
have data that you don't want to be visible to others in the bug
report, you can use FTP to transfer it to
`ftp://ftp.mysql.com/pub/mysql/upload/'. If the information is
really top secret and you don't want to show it even to us, go
ahead and provide an example using other names, but please regard
this as the last choice.
* Include all the options given to the relevant programs, if
possible. For example, indicate the options that you use when you
start the `mysqld' server, as well as the options that you use to
run any MySQL client programs. The options to programs such as
`mysqld' and `mysql', and to the `configure' script, are often key
to resolving problems and are very relevant. It is never a bad
idea to include them. If your problem involves a program written
in a language such as Perl or PHP, please include the language
processor's version number, as well as the version for any modules
that the program uses. For example, if you have a Perl script that
uses the `DBI' and `DBD::mysql' modules, include the version
numbers for Perl, `DBI', and `DBD::mysql'.
* If your question is related to the privilege system, please
include the output of `mysqlaccess', the output of `mysqladmin
reload', and all the error messages you get when trying to
connect. When you test your privileges, you should first run
`mysqlaccess'. After this, execute `mysqladmin reload version'
and try to connect with the program that gives you trouble.
`mysqlaccess' can be found in the `bin' directory under your MySQL
installation directory.
* If you have a patch for a bug, do include it. But don't assume
that the patch is all we need, or that we can use it, if you don't
provide some necessary information such as test cases showing the
bug that your patch fixes. We might find problems with your patch
or we might not understand it at all. If so, we can't use it.
If we can't verify the exact purpose of the patch, we won't use
it. Test cases help us here. Show that the patch handles all the
situations that may occur. If we find a borderline case (even a
rare one) where the patch won't work, it may be useless.
* Guesses about what the bug is, why it occurs, or what it depends
on are usually wrong. Even the MySQL team can't guess such things
without first using a debugger to determine the real cause of a
bug.
* Indicate in your bug report that you have checked the reference
manual and mail archive so that others know you have tried to
solve the problem yourself.
* If the problem is that your data appears corrupt or you get errors
when you access a particular table, you should first check your
tables and then try to repair them with `CHECK TABLE' and `REPAIR
TABLE' or with `myisamchk'. See *Note database-administration::.
If you are running Windows, please verify the value of
`lower_case_table_names' using the `SHOW VARIABLES LIKE
'lower_case_table_names'' command. This variable affects how the
server handles lettercase of database and table names. Its effect
for a given value should be as described in *Note
name-case-sensitivity::.
* If you often get corrupted tables, you should try to find out when
and why this happens. In this case, the error log in the MySQL
data directory may contain some information about what happened.
(This is the file with the `.err' suffix in the name.) See *Note
error-log::. Please include any relevant information from this
file in your bug report. Normally `mysqld' should _never_ crash a
table if nothing killed it in the middle of an update. If you can
find the cause of `mysqld' dying, it's much easier for us to
provide you with a fix for the problem. See *Note
what-is-crashing::.
* If possible, download and install the most recent version of MySQL
Server and check whether it solves your problem. All versions of
the MySQL software are thoroughly tested and should work without
problems. We believe in making everything as backward-compatible
as possible, and you should be able to switch MySQL versions
without difficulty. See *Note which-version::.
If you have no Web access and cannot report a bug by visiting
`http://bugs.mysql.com/', you can use the `mysqlbug' script to generate
a bug report (or a report about any problem). `mysqlbug' helps you
generate a report by determining much of the following information
automatically, but if something important is missing, please include it
with your message. `mysqlbug' can be found in the `scripts' directory
(source distribution) and in the `bin' directory under your MySQL
installation directory (binary distribution).
File: manual.info, Node: compatibility, Prev: bug-reports, Up: introduction
1.9 MySQL Standards Compliance
==============================
* Menu:
* standards:: What Standards MySQL Follows
* sql-mode:: Selecting SQL Modes
* ansi-mode:: Running MySQL in ANSI Mode
* extensions-to-ansi:: MySQL Extensions to Standard SQL
* differences-from-ansi:: MySQL Differences from Standard SQL
* constraints:: How MySQL Deals with Constraints
This section describes how MySQL relates to the ANSI/ISO SQL standards.
MySQL Server has many extensions to the SQL standard, and here you can
find out what they are and how to use them. You can also find
information about functionality missing from MySQL Server, and how to
work around some of the differences.
The SQL standard has been evolving since 1986 and several versions
exist. In this manual, `SQL-92' refers to the standard released in
1992, `SQL:1999' refers to the standard released in 1999, and
`SQL:2003' refers to the current version of the standard. We use the
phrase `the SQL standard' or `standard SQL' to mean the current version
of the SQL Standard at any time.
One of our main goals with the product is to continue to work toward
compliance with the SQL standard, but without sacrificing speed or
reliability. We are not afraid to add extensions to SQL or support for
non-SQL features if this greatly increases the usability of MySQL
Server for a large segment of our user base. The `HANDLER' interface
is an example of this strategy. See *Note handler::.
We continue to support transactional and non-transactional databases to
satisfy both mission-critical 24/7 usage and heavy Web or logging usage.
MySQL Server was originally designed to work with medium-sized
databases (10-100 million rows, or about 100MB per table) on small
computer systems. Today MySQL Server handles terabyte-sized databases,
but the code can also be compiled in a reduced version suitable for
hand-held and embedded devices. The compact design of the MySQL server
makes development in both directions possible without any conflicts in
the source tree.
Currently, we are not targeting real-time support, although MySQL
replication capabilities offer significant functionality.
MySQL supports high-availability database clustering using the
`NDBCluster' storage engine. See *Note ndbcluster::.
XML support is to be implemented in a future version of the database
server.
File: manual.info, Node: standards, Next: sql-mode, Prev: compatibility, Up: compatibility
1.9.1 What Standards MySQL Follows
----------------------------------
Our aim is to support the full ANSI/ISO SQL standard, but without
making concessions to speed and quality of the code.
ODBC levels 0-3.51.
File: manual.info, Node: sql-mode, Next: ansi-mode, Prev: standards, Up: compatibility
1.9.2 Selecting SQL Modes
-------------------------
The MySQL server can operate in different SQL modes, and can apply
these modes differentially for different clients. This capability
enables each application to tailor the server's operating mode to its
own requirements.
SQL modes control aspects of server operation such as what SQL syntax
MySQL should support and what kind of data validation checks it should
perform. This makes it easier to use MySQL in different environments
and to use MySQL together with other database servers.
You can set the default SQL mode by starting `mysqld' with the
-sql-mode="MODE_VALUE" option. Beginning with MySQL 4.1, you can also
change the mode at runtime by setting the `sql_mode' system variable
with a `SET [SESSION|GLOBAL] sql_mode='MODE_VALUE'' statement.
For more information on setting the SQL mode, see *Note
server-sql-mode::.
File: manual.info, Node: ansi-mode, Next: extensions-to-ansi, Prev: sql-mode, Up: compatibility
1.9.3 Running MySQL in ANSI Mode
--------------------------------
You can tell `mysqld' to run in ANSI mode with the -ansi startup
option. Running the server in ANSI mode is the same as starting it with
the following options:
--transaction-isolation=SERIALIZABLE --sql-mode=ANSI
As of MySQL 4.1.1, you can achieve the same effect at runtime by
executing these two statements:
SET GLOBAL TRANSACTION ISOLATION LEVEL SERIALIZABLE;
SET GLOBAL sql_mode = 'ANSI';
You can see that setting the `sql_mode' system variable to `'ANSI''
enables all SQL mode options that are relevant for ANSI mode as follows:
mysql> SET GLOBAL sql_mode='ANSI';
mysql> SELECT @@global.sql_mode;
-> 'REAL_AS_FLOAT,PIPES_AS_CONCAT,ANSI_QUOTES,IGNORE_SPACE,ANSI'
Note that running the server in ANSI mode with -ansi is not quite the
same as setting the SQL mode to `'ANSI''. The -ansi option affects the
SQL mode and also sets the transaction isolation level. Setting the SQL
mode to `'ANSI'' has no effect on the isolation level.
See *Note server-options::, and *Note sql-mode::.
File: manual.info, Node: extensions-to-ansi, Next: differences-from-ansi, Prev: ansi-mode, Up: compatibility
1.9.4 MySQL Extensions to Standard SQL
--------------------------------------
MySQL Server supports some extensions that you probably won't find in
other SQL DBMSs. Be warned that if you use them, your code won't be
portable to other SQL servers. In some cases, you can write code that
includes MySQL extensions, but is still portable, by using comments of
the following form:
/*! MYSQL-SPECIFIC CODE */
In this case, MySQL Server parses and executes the code within the
comment as it would any other SQL statement, but other SQL servers will
ignore the extensions. For example, MySQL Server recognizes the
`STRAIGHT_JOIN' keyword in the following statement, but other servers
will not:
SELECT /*! STRAIGHT_JOIN */ col1 FROM table1,table2 WHERE ...
If you add a version number after the ‘`!'’ character, the syntax
within the comment is executed only if the MySQL version is greater
than or equal to the specified version number. The `TEMPORARY' keyword
in the following comment is executed only by servers from MySQL 3.23.02
or higher:
CREATE /*!32302 TEMPORARY */ TABLE t (a INT);
The following descriptions list MySQL extensions, organized by category.
* Organization of data on disk
MySQL Server maps each database to a directory under the MySQL
data directory, and maps tables within a database to filenames in
the database directory. This has a few implications:
* Database and table names are case sensitive in MySQL Server
on operating systems that have case-sensitive filenames (such
as most Unix systems). See *Note name-case-sensitivity::.
* You can use standard system commands to back up, rename,
move, delete, and copy tables that are managed by the
`MyISAM' storage engine. For example, it is possible to
rename a `MyISAM' table by renaming the `.MYD', `.MYI', and
`.frm' files to which the table corresponds. (Nevertheless, it
is preferable to use `RENAME TABLE' or `ALTER TABLE ...
RENAME' and let the server rename the files.)
Database and table names cannot contain pathname separator
characters (‘`/'’, ‘`\'’).
* General language syntax
* By default, strings can be enclosed by either ‘`"'’ or
‘`''’, not just by ‘`''’. (If the `ANSI_QUOTES' SQL
mode is enabled, strings can be enclosed only by ‘`''’
and the server interprets strings enclosed by ‘`"'’ as
identifiers.)
* ‘`\'’ is the escape character in strings.
* In SQL statements, you can access tables from different
databases with the DB_NAME.TBL_NAME syntax. Some SQL servers
provide the same functionality but call this `User space'.
MySQL Server doesn't support tablespaces such as used in
statements like this: `CREATE TABLE ralph.my_table ... IN
my_tablespace'.
* SQL statement syntax
* The `ANALYZE TABLE', `CHECK TABLE', `OPTIMIZE TABLE', and
`REPAIR TABLE' statements.
* The `CREATE DATABASE', `DROP DATABASE', and `ALTER DATABASE'
statements. See *Note create-database::, *Note
drop-database::, and *Note alter-database::.
* The `DO' statement.
* `EXPLAIN SELECT' to obtain a description of how tables are
processed by the query optimizer.
* The `FLUSH' and `RESET' statements.
* The `SET' statement. See *Note set-option::.
* The `SHOW' statement. See *Note show::. As of MySQL 5.0, the
information produced by many of the MySQL-specific `SHOW'
statements can be obtained in more standard fashion by using
`SELECT' to query `INFORMATION_SCHEMA'. See *Note
information-schema::.
* Use of `LOAD DATA INFILE'. In many cases, this syntax is
compatible with Oracle's `LOAD DATA INFILE'. See *Note
load-data::.
* Use of `RENAME TABLE'. See *Note rename-table::.
* Use of `REPLACE' instead of `DELETE' plus `INSERT'. See *Note
replace::.
* Use of `CHANGE COL_NAME', `DROP COL_NAME', or `DROP INDEX',
`IGNORE' or `RENAME' in `ALTER TABLE' statements. Use of
multiple `ADD', `ALTER', `DROP', or `CHANGE' clauses in an
`ALTER TABLE' statement. See *Note alter-table::.
* Use of index names, indexes on a prefix of a column, and use
of `INDEX' or `KEY' in `CREATE TABLE' statements. See *Note
create-table::.
* Use of `TEMPORARY' or `IF NOT EXISTS' with `CREATE TABLE'.
* Use of `IF EXISTS' with `DROP TABLE' and `DROP DATABASE'.
* The capability of dropping multiple tables with a single
`DROP TABLE' statement.
* The `ORDER BY' and `LIMIT' clauses of the `UPDATE' and
`DELETE' statements.
* `INSERT INTO `tbl_name' SET COL_NAME = ...' syntax.
* The `DELAYED' clause of the `INSERT' and `REPLACE' statements.
* The `LOW_PRIORITY' clause of the `INSERT', `REPLACE',
`DELETE', and `UPDATE' statements.
* Use of `INTO OUTFILE' or `INTO DUMPFILE' in `SELECT'
statements. See *Note select::.
* Options such as `STRAIGHT_JOIN' or `SQL_SMALL_RESULT' in
`SELECT' statements.
* You don't need to name all selected columns in the `GROUP BY'
clause. This gives better performance for some very specific,
but quite normal queries. See *Note
group-by-functions-and-modifiers::.
* You can specify `ASC' and `DESC' with `GROUP BY', not just
with `ORDER BY'.
* The ability to set variables in a statement with the `:='
assignment operator:
mysql> SELECT @a:=SUM(total),@b=COUNT(*),@a/@b AS avg
-> FROM test_table;
mysql> SELECT @t1:=(@t2:=1)+@t3:=4,@t1,@t2,@t3;
* Data types
* The `MEDIUMINT', `SET', and `ENUM' data types, and the
various `BLOB' and `TEXT' data types.
* The `AUTO_INCREMENT', `BINARY', `NULL', `UNSIGNED', and
`ZEROFILL' data type attributes.
* Functions and operators
* To make it easier for users who migrate from other SQL
environments, MySQL Server supports aliases for many
functions. For example, all string functions support both
standard SQL syntax and ODBC syntax.
* MySQL Server understands the `||' and `&&' operators to mean
logical OR and AND, as in the C programming language. In MySQL
Server, `||' and `OR' are synonyms, as are `&&' and `AND'.
Because of this nice syntax, MySQL Server doesn't support the
standard SQL `||' operator for string concatenation; use
`CONCAT()' instead. Because `CONCAT()' takes any number of
arguments, it's easy to convert use of the `||' operator to
MySQL Server.
* Use of `COUNT(DISTINCT VALUE_LIST)' where VALUE_LIST has more
than one element.
* String comparisons are case-insensitive by default, with sort
ordering determined by collation of the current character
set, which is `latin1' (cp1252 West European) by default. If
you don't like this, you should declare your columns with the
`BINARY' attribute or use the `BINARY' cast, which causes
comparisons to be done using the underlying character code
values rather then a lexical ordering.
* The `%' operator is a synonym for `MOD()'. That is, `N % M'
is equivalent to `MOD(N,M)'. `%' is supported for C
programmers and for compatibility with PostgreSQL.
* The `=', `<>', `<=',`<', `>=',`>', `<<', `>>', `<=>', `AND',
`OR', or `LIKE' operators may be used in expressions in the
output column list (to the left of the `FROM') in `SELECT'
statements. For example:
mysql> SELECT col1=1 AND col2=2 FROM my_table;
* The `LAST_INSERT_ID()' function returns the most recent
`AUTO_INCREMENT' value. See *Note information-functions::.
* `LIKE' is allowed on numeric values.
* The `REGEXP' and `NOT REGEXP' extended regular expression
operators.
* `CONCAT()' or `CHAR()' with one argument or more than two
arguments. (In MySQL Server, these functions can take a
variable number of arguments.)
* The `BIT_COUNT()', `CASE', `ELT()', `FROM_DAYS()',
`FORMAT()', `IF()', `PASSWORD()', `ENCRYPT()', `MD5()',
`ENCODE()', `DECODE()', `PERIOD_ADD()', `PERIOD_DIFF()',
`TO_DAYS()', and `WEEKDAY()' functions.
* Use of `TRIM()' to trim substrings. Standard SQL supports
removal of single characters only.
* The `GROUP BY' functions `STD()', `BIT_OR()', `BIT_AND()',
`BIT_XOR()', and `GROUP_CONCAT()'. See *Note
group-by-functions-and-modifiers::.
For a prioritized list indicating when new extensions are added to
MySQL Server, you should consult the online MySQL development roadmap
at `http://dev.mysql.com/doc/mysql/en/roadmap.html'.
File: manual.info, Node: differences-from-ansi, Next: constraints, Prev: extensions-to-ansi, Up: compatibility
1.9.5 MySQL Differences from Standard SQL
-----------------------------------------
* Menu:
* ansi-diff-subqueries:: Subquery Support
* ansi-diff-select-into-table:: `SELECT INTO TABLE'
* ansi-diff-transactions:: Transactions and Atomic Operations
* ansi-diff-triggers:: Stored Routines and Triggers
* ansi-diff-foreign-keys:: Foreign Keys
* ansi-diff-views:: Views
* ansi-diff-comments:: '`--'' as the Start of a Comment
We try to make MySQL Server follow the ANSI SQL standard and the ODBC
SQL standard, but MySQL Server performs operations differently in some
cases:
* For `VARCHAR' columns, trailing spaces are removed when the value
is stored. (This is fixed in MySQL 5.0.3). See *Note bugs::.
* In some cases, `CHAR' columns are silently converted to `VARCHAR'
columns when you define a table or alter its structure. (This is
fixed in MySQL 5.0.3). See *Note silent-column-changes::.
* There are several differences between the MySQL and standard SQL
privilege systems. For example, in MySQL, privileges for a table
are not automatically revoked when you delete a table. You must
explicitly issue a `REVOKE' statement to revoke privileges for a
table. For more information, see *Note revoke::.
* The `CAST()' function does not support cast to `REAL' or `BIGINT'.
See *Note cast-functions::.
* Standard SQL requires that a `HAVING' clause in a `SELECT'
statement be able to refer to columns in the `GROUP BY' clause.
This cannot be done before MySQL 5.0.2.
File: manual.info, Node: ansi-diff-subqueries, Next: ansi-diff-select-into-table, Prev: differences-from-ansi, Up: differences-from-ansi
1.9.5.1 Subquery Support
........................
MySQL 4.1 and up supports subqueries and derived tables. A `subquery'
is a `SELECT' statement nested within another statement. A `derived
table' (an unnamed view) is a subquery in the `FROM' clause of another
statement. See *Note subqueries::.
For MySQL versions older than 4.1, most subqueries can be rewritten
using joins or other methods. See *Note rewriting-subqueries::, for
examples that show how to do this.
File: manual.info, Node: ansi-diff-select-into-table, Next: ansi-diff-transactions, Prev: ansi-diff-subqueries, Up: differences-from-ansi
1.9.5.2 `SELECT INTO TABLE'
...........................
MySQL Server doesn't support the `SELECT ... INTO TABLE' Sybase SQL
extension. Instead, MySQL Server supports the `INSERT INTO ... SELECT'
standard SQL syntax, which is basically the same thing. See *Note
insert-select::. For example:
INSERT INTO tbl_temp2 (fld_id)
SELECT tbl_temp1.fld_order_id
FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;
Alternatively, you can use `SELECT ... INTO OUTFILE' or `CREATE TABLE
... SELECT'.
As of MySQL 5.0, you can use `SELECT ... INTO' with user-defined
variables. The same syntax can also be used inside stored routines
using cursors and local variables. See *Note select-into-statement::.
File: manual.info, Node: ansi-diff-transactions, Next: ansi-diff-triggers, Prev: ansi-diff-select-into-table, Up: differences-from-ansi
1.9.5.3 Transactions and Atomic Operations
..........................................
MySQL Server (version 3.23-max and all versions 4.0 and above) supports
transactions with the `InnoDB' and `BDB' transactional storage engines.
`InnoDB' provides _full_ `ACID' compliance. See *Note
storage-engines::. For information about `InnoDB' differences from
standard SQL with regard to treatment of transaction errors, see *Note
innodb-error-handling::.
The other non-transactional storage engines in MySQL Server (such as
`MyISAM') follow a different paradigm for data integrity called `atomic
operations.' In transactional terms, `MyISAM' tables effectively always
operate in `AUTOCOMMIT=1' mode. Atomic operations often offer
comparable integrity with higher performance.
Because MySQL Server supports both paradigms, you can decide whether
your applications are best served by the speed of atomic operations or
the use of transactional features. This choice can be made on a
per-table basis.
As noted, the trade-off for transactional versus non-transactional
storage engines lies mostly in performance. Transactional tables have
significantly higher memory and disk space requirements, and more CPU
overhead. On the other hand, transactional storage engines such as
`InnoDB' also offer many significant features. MySQL Server's modular
design allows the concurrent use of different storage engines to suit
different requirements and deliver optimum performance in all
situations.
But how do you use the features of MySQL Server to maintain rigorous
integrity even with the non-transactional `MyISAM' tables, and how do
these features compare with the transactional storage engines?
* If your applications are written in a way that is dependent on
being able to call `ROLLBACK' rather than `COMMIT' in critical
situations, transactions are more convenient. Transactions also
ensure that unfinished updates or corrupting activities are not
committed to the database; the server is given the opportunity to
do an automatic rollback and your database is saved.
If you use non-transactional tables, MySQL Server in almost all
cases allows you to resolve potential problems by including simple
checks before updates and by running simple scripts that check the
databases for inconsistencies and automatically repair or warn if
such an inconsistency occurs. Note that just by using the MySQL
log or even adding one extra log, you can normally fix tables
perfectly with no data integrity loss.
* More often than not, critical transactional updates can be
rewritten to be atomic. Generally speaking, all integrity problems
that transactions solve can be done with `LOCK TABLES' or atomic
updates, ensuring that there are no automatic aborts from the
server, which is a common problem with transactional database
systems.
* To be safe with MySQL Server, regardless of whether you use
transactional tables, you only need to have backups and have
binary logging turned on. When that is true, you can recover from
any situation that you could with any other transactional database
system. It is always good to have backups, regardless of which
database system you use.
The transactional paradigm has its benefits and its drawbacks. Many
users and application developers depend on the ease with which they can
code around problems where an abort appears to be necessary, or is
necessary. However, even if you are new to the atomic operations
paradigm, or more familiar with transactions, do consider the speed
benefit that non-transactional tables can offer on the order of three to
five times the speed of the fastest and most optimally tuned
transactional tables.
In situations where integrity is of highest importance, MySQL Server
offers transaction-level reliability and integrity even for
non-transactional tables. If you lock tables with `LOCK TABLES', all
updates stall until integrity checks are made. If you obtain a `READ
LOCAL' lock (as opposed to a write lock) for a table that allows
concurrent inserts at the end of the table, reads are allowed, as are
inserts by other clients. The newly inserted records are not be seen by
the client that has the read lock until it releases the lock. With
`INSERT DELAYED', you can write inserts that go into a local queue
until the locks are released, without having the client wait for the
insert to complete. See *Note concurrent-inserts::, and *Note
insert-delayed::.
`Atomic,' in the sense that we mean it, is nothing magical. It only
means that you can be sure that while each specific update is running,
no other user can interfere with it, and there can never be an
automatic rollback (which can happen with transactional tables if you
are not very careful). MySQL Server also guarantees that there are no
dirty reads.
Following are some techniques for working with non-transactional tables:
* Loops that need transactions normally can be coded with the help
of `LOCK TABLES', and you don't need cursors to update records on
the fly.
* To avoid using `ROLLBACK', you can employ the following strategy:
1. Use `LOCK TABLES' to lock all the tables you want to access.
2. Test the conditions that must be true before performing the
update.
3. Update if the conditions are satisfied.
4. Use `UNLOCK TABLES' to release your locks.
This is usually a much faster method than using transactions with
possible rollbacks, although not always. The only situation this
solution doesn't handle is when someone kills the threads in the
middle of an update. In that case, all locks are released but some
of the updates may not have been executed.
* You can also use functions to update records in a single
operation. You can get a very efficient application by using the
following techniques:
* Modify columns relative to their current value.
* Update only those columns that actually have changed.
For example, when we are updating customer information, we update
only the customer data that has changed and test only that none of
the changed data, or data that depends on the changed data, has
changed compared to the original row. The test for changed data is
done with the `WHERE' clause in the `UPDATE' statement. If the
record wasn't updated, we give the client a message: `Some of the
data you have changed has been changed by another user.' Then we
show the old row versus the new row in a window so that the user
can decide which version of the customer record to use.
This gives us something that is similar to column locking but is
actually even better because we only update some of the columns,
using values that are relative to their current values. This means
that typical `UPDATE' statements look something like these:
UPDATE tablename SET pay_back=pay_back+125;
UPDATE customer
SET
customer_date='current_date',
address='new address',
phone='new phone',
money_owed_to_us=money_owed_to_us-125
WHERE
customer_id=id AND address='old address' AND phone='old phone';
This is very efficient and works even if another client has
changed the values in the `pay_back' or `money_owed_to_us' columns.
* In many cases, users have wanted `LOCK TABLES' or `ROLLBACK' for
the purpose of managing unique identifiers. This can be handled
much more efficiently without locking or rolling back by using an
`AUTO_INCREMENT' column and either the `LAST_INSERT_ID()' SQL
function or the `mysql_insert_id()' C API function. See *Note
information-functions::, and *Note mysql-insert-id::.
You can generally code around the need for row-level locking. Some
situations really do need it, and `InnoDB' tables support row-level
locking. Otherwise, with `MyISAM' tables, you can use a flag
column in the table and do something like the following:
UPDATE TBL_NAME SET row_flag=1 WHERE id=ID;
MySQL returns `1' for the number of affected rows if the row was
found and `row_flag' wasn't `1' in the original row. You can think
of this as though MySQL Server changed the preceding statement to:
UPDATE TBL_NAME SET row_flag=1 WHERE id=ID AND row_flag <> 1;
File: manual.info, Node: ansi-diff-triggers, Next: ansi-diff-foreign-keys, Prev: ansi-diff-transactions, Up: differences-from-ansi
1.9.5.4 Stored Routines and Triggers
....................................
Stored procedures and functions are implemented beginning with MySQL
5.0. See *Note stored-procedures::.
Basic trigger functionality is implemented beginning with MySQL 5.0.2,
with further development planned for MySQL 5.1. See *Note triggers::.
File: manual.info, Node: ansi-diff-foreign-keys, Next: ansi-diff-views, Prev: ansi-diff-triggers, Up: differences-from-ansi
1.9.5.5 Foreign Keys
....................
In MySQL Server 3.23.44 and up, the `InnoDB' storage engine supports
checking of foreign key constraints, including `CASCADE', `ON DELETE',
and `ON UPDATE'. See *Note innodb-foreign-key-constraints::.
For storage engines other than `InnoDB', MySQL Server parses the
`FOREIGN KEY' syntax in `CREATE TABLE' statements, but does not use or
store it. In the future, the implementation will be extended to store
this information in the table specification file so that it may be
retrieved by `mysqldump' and ODBC. At a later stage, foreign key
constraints will be implemented for `MyISAM' tables as well.
Foreign key enforcement offers several benefits to database developers:
* Assuming proper design of the relationships, foreign key
constraints make it more difficult for a programmer to introduce
an inconsistency into the database.
* Centralized checking of constraints by the database server makes
it unnecessary to perform these checks on the application side.
This eliminates the possibility that different applications may
not all check the constraints in the same way.
* Using cascading updates and deletes can simplify the application
code.
* Properly designed foreign key rules aid in documenting
relationships between tables.
Do keep in mind that these benefits come at the cost of additional
overhead for the database server to perform the necessary checks.
Additional checking by the server affects performance, which for some
applications may be sufficiently undesirable as to be avoided if
possible. (Some major commercial applications have coded the foreign
key logic at the application level for this reason.)
MySQL gives database developers the choice of which approach to use. If
you don't need foreign keys and want to avoid the overhead associated
with enforcing referential integrity, you can choose another storage
engine instead, such as `MyISAM'. (For example, the `MyISAM' storage
engine offers very fast performance for applications that perform only
`INSERT' and `SELECT' operations. In this case, the table has no holes
in the middle and the inserts can be performed concurrently with
retrievals. See *Note concurrent-inserts::.)
If you choose not to take advantage of referential integrity checks,
keep the following considerations in mind:
* In the absence of server-side foreign key relationship checking,
the application itself must handle relationship issues. For
example, it must take care to insert rows into tables in the
proper order, and to avoid creating orphaned child records. It
must also be able to recover from errors that occur in the middle
of multiple-record insert operations.
* If `ON DELETE' is the only referential integrity capability an
application needs, you can achieve a similar effect as of MySQL
Server 4.0 by using multiple-table `DELETE' statements to delete
rows from many tables with a single statement. See *Note delete::.
* A workaround for the lack of `ON DELETE' is to add the appropriate
`DELETE' statements to your application when you delete records
from a table that has a foreign key. In practice, this is often as
quick as using foreign keys and is more portable.
Be aware that the use of foreign keys can sometimes lead to problems:
* Foreign key support addresses many referential integrity issues,
but it is still necessary to design key relationships carefully to
avoid circular rules or incorrect combinations of cascading
deletes.
* It is not uncommon for a DBA to create a topology of relationships
that makes it difficult to restore individual tables from a
backup. (MySQL alleviates this difficulty by allowing you to
temporarily disable foreign key checks when reloading a table that
depends on other tables. See *Note
innodb-foreign-key-constraints::. As of MySQL 4.1.1, `mysqldump'
generates dump files that take advantage of this capability
automatically when they are reloaded.)
Note that foreign keys in SQL are used to check and enforce referential
integrity, not to join tables. If you want to get results from multiple
tables from a `SELECT' statement, you do this by performing a join
between them:
SELECT * FROM t1 INNER JOIN t2 ON t1.id = t2.id;
See *Note join::, and *Note example-foreign-keys::.
The `FOREIGN KEY' syntax without `ON DELETE ...' is often used by ODBC
applications to produce automatic `WHERE' clauses.
File: manual.info, Node: ansi-diff-views, Next: ansi-diff-comments, Prev: ansi-diff-foreign-keys, Up: differences-from-ansi
1.9.5.6 Views
.............
Views (including updatable views) are implemented beginning with MySQL
Server 5.0.1. See *Note views::.
Views are useful for allowing users to access a set of relations
(tables) as if it were a single table, and limiting their access to
just that. Views can also be used to restrict access to rows (a subset
of a particular table). For access control to columns, you can also use
the sophisticated privilege system in MySQL Server. See *Note
privilege-system::.
In designing an implementation of views, our ambitious goal, as much as
is possible within the confines of SQL, has been full compliance with
`Codd's Rule #6' for relational database systems: `All views that are
theoretically updatable, should in practice also be updatable.'
File: manual.info, Node: ansi-diff-comments, Prev: ansi-diff-views, Up: differences-from-ansi
1.9.5.7 '`--'' as the Start of a Comment
........................................
Standard SQL uses the C syntax `/* this is a comment */' for comments,
and MySQL Server supports this syntax as well. MySQL also support
extensions to this syntax that allow MySQL-specific SQL to be embedded
in the comment, as described in *Note comments::.
Standard SQL uses ‘-’ as a start-comment sequence. MySQL Server uses
‘`#'’ as the start comment character. MySQL Server 3.23.3 and up
also supports a variant of the ‘`--'’ comment style. That is, the
‘`--'’ start-comment sequence must be followed by a space (or by a
control character such as a newline). The space is required to prevent
problems with automatically generated SQL queries that use constructs
such as the following, where we automatically insert the value of the
payment for `payment':
UPDATE account SET credit=credit-payment
Consider about what happens if `payment' has a negative value such as
`-1':
UPDATE account SET credit=credit--1
`credit--1' is a legal expression in SQL, but ‘`--'’ is interpreted
as the start of a comment, part of the expression is discarded. The
result is a statement that has a completely different meaning than
intended:
UPDATE account SET credit=credit
The statement produces no change in value at all. This illustrates that
allowing comments to start with ‘`--'’ can have serious
consequences.
Using our implementation requires a space following the ‘`--'’ in
order for it to be recognized as a start-comment sequence in MySQL
Server 3.23.3 and newer. Therefore, `credit--1' is safe to use.
Another safe feature is that the `mysql' command-line client ignores
lines that start with ‘`--'’.
The following information is relevant only if you are running a MySQL
version earlier than 3.23.3:
If you have an SQL script in a text file that contains ‘`--'’
comments, you should use the `replace' utility as follows to convert
the comments to use ‘`#'’ characters before executing the script:
shell> replace " --" " #" < text-file-with-funny-comments.sql \
| mysql DB_NAME
That is safer than executing the script in the usual way:
shell> mysql DB_NAME < text-file-with-funny-comments.sql
You can also edit the script file `in place' to change the ‘`--'’
comments to ‘`#'’ comments:
shell> replace " --" " #" -- text-file-with-funny-comments.sql
Change them back with this command:
shell> replace " #" " --" -- text-file-with-funny-comments.sql
See *Note replace-utility::.
File: manual.info, Node: constraints, Prev: differences-from-ansi, Up: compatibility
1.9.6 How MySQL Deals with Constraints
--------------------------------------
* Menu:
* constraint-primary-key:: `PRIMARY KEY' and `UNIQUE' Index Constraints
* constraint-invalid-data:: Constraints on Invalid Data
* constraint-enum:: `ENUM' and `SET' Constraints
MySQL allows you to work both with transactional tables that allow
rollback and with non-transactional tables that do not. Because of
this, constraint handling is a bit different in MySQL than in other
DBMSs. We must handle the case when you have inserted or updated a lot
of rows in a non-transactional table for which changes cannot be rolled
back when an error occurs.
The basic philosophy is that MySQL Server tries to produce an error for
anything that it can detect while parsing a statement to be executed,
and tries to recover from any errors that occur while executing the
statement. We do this in most cases, but not yet for all.
The options MySQL has when an error occurs are to stop the statement in
the middle or to recover as well as possible from the problem and
continue. By default, the server follows the latter course. This means,
for example, that the server may coerce illegal values to the closest
legal values.
Beginning with MySQL 5.0.2, several SQL mode options are available to
provide greater control over handling of bad data values and whether to
continue statement execution or abort when errors occur. Using these
options, you can configure MySQL Server to act in a more traditional
fashion that is like other DBMSs that reject improper input. The SQL
mode can be set globally at server startup to affect all clients.
Individual clients can set the SQL mode at runtime, which enables each
client to select the behavior most appropriate for its requirements.
See *Note server-sql-mode::.
The following sections describe how MySQL Server handles different
types of constraints.
File: manual.info, Node: constraint-primary-key, Next: constraint-invalid-data, Prev: constraints, Up: constraints
1.9.6.1 `PRIMARY KEY' and `UNIQUE' Index Constraints
....................................................
Normally, an error occurs when you try to `INSERT' or `UPDATE' a row
that causes a primary key, unique key, or foreign key violation. If you
are using a transactional storage engine such as `InnoDB', MySQL
automatically rolls back the statement. If you are using a
non-transactional storage engine, MySQL stops processing the statement
at the row for which the error occurred and leaves any remaining rows
unprocessed.
If you want to ignore such key violations, MySQL supports an `IGNORE'
keyword for `INSERT' and `UPDATE'. In this case, MySQL ignores any key
violations and continues processing with the next row. See *Note
insert::, and *Note update::.
You can get information about the number of rows actually inserted or
updated with the `mysql_info()' C API function. In MySQL 4.1 and up,
you also can use the `SHOW WARNINGS' statement. See *Note mysql-info::,
and *Note show-warnings::.
Currently, only `InnoDB' tables support foreign keys. See *Note
innodb-foreign-key-constraints::. Foreign key support in `MyISAM'
tables is scheduled for implementation in MySQL 5.2. See *Note
roadmap::.
File: manual.info, Node: constraint-invalid-data, Next: constraint-enum, Prev: constraint-primary-key, Up: constraints
1.9.6.2 Constraints on Invalid Data
...................................
Before MySQL 5.0.2, MySQL is forgiving of illegal or improper data
values and coerces them to legal values for data entry. In MySQL 5.0.2
and up, that remains the default behavior, but you can change the
server SQL mode to select more traditional treatment of bad values such
that the server rejects them and aborts the statement in which they
occur. *Note server-sql-mode::.
This section describes the default (forgiving) behavior of MySQL, as
well as the newer strict SQL mode and how it differs.
If you are not using strict mode, then whenever you insert an
`incorrect' value into a column, such as a `NULL' into a `NOT NULL'
column or a too-large numeric value into a numeric column, MySQL sets
the column to the `best possible value' instead of producing an error:
The following rules describe in more detail how this works:
* If you try to store an out of range value into a numeric column,
MySQL Server instead stores zero, the smallest possible value, or
the largest possible value, whichever is closest to the invalid
value.
* For strings, MySQL stores either the empty string or as much of
the string as can be stored in the column.
* If you try to store a string that doesn't start with a number into
a numeric column, MySQL Server stores 0.
* Invalid values for `ENUM' and `SET' columns ae handled as
described in *Note constraint-enum::.
* MySQL allows you to store certain incorrect date values into
`DATE' and `DATETIME' columns (such as `'2000-02-31'' or
`'2000-02-00''). The idea is that it's not the job of the SQL
server to validate dates. If MySQL can store a date value and
retrieve exactly the same value, MySQL stores it as given. If the
date is totally wrong (outside the server's ability to store it),
the special `zero' date value `'0000-00-00'' is stored in the
column instead.
* If you try to store `NULL' into a column that doesn't take `NULL'
values, an error occurs for single-row `INSERT' statements. For
multiple-row `INSERT' statements or for `INSERT INTO ... SELECT'
statements, MySQL Server stores the implicit default value for the
column data type. In general, this is `0' for numeric types, the
empty string (`''') for string types, and the `zero' value for
date and time types. Implicit default values are discussed in
*Note data-type-defaults::.
* If an `INSERT' statement specifies no value for a column, MySQL
inserts its default value if the column definition includes an
explicit `DEFAULT' clause. If the definition has no such `DEFAULT'
clause, MySQL inserts the implicit default value for the column
data type.
The reason for using the preceding rules in non-strict mode is that we
can't check these conditions until