Table of Contents
The MySQL® 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 Section 1.4.3, “The Main Features of MySQL”.
For future plans, see Section 1.5, “MySQL Development Roadmap”.
For installation instructions, see Chapter 2, Installing and Upgrading MySQL. For information about upgrading MySQL, see Section 2.4.17, “Upgrading MySQL”.
For a tutorial introduction to the MySQL Database Server, see Chapter 3, Tutorial.
For information about configuring and administering MySQL Server, see Chapter 5, MySQL Server Administration.
For information about setting up replication servers, see Chapter 15, Replication.
For answers to a number of questions that are often asked concerning the MySQL Database Server and its capabilities, see Appendix A, MySQL 5.0 Frequently Asked Questions.
For a list of currently known bugs and misfeatures, see Section B.1.8, “Known Issues in MySQL”.
For a list of all the contributors to this project, see Appendix G, Credits.
For a history of new features and bugfixes, see Appendix E, MySQL Change History.
For tips on porting the MySQL Database Software to new architectures or operating systems, see MySQL Internals: Porting.
For benchmarking information, see the
sql-bench benchmarking directory in your
MySQL distribution.
To report errors (often called “bugs”), please use the instructions at Section 1.7, “How to Report Bugs or Problems”.
If you have found a sensitive security bug in MySQL Server, please
let us know immediately by sending an email message to
<security@mysql.com>.
This is the Reference Manual for the MySQL Database System,
version 5.0, through release 5.0.56. 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://dev.mysql.com/tech-resources/sources.html. You can check out a copy of the documentation repository with this command:
svn checkout http://svn.mysql.com/svnpublic/mysqldoc/
If you have questions about using MySQL, you can ask them using
our mailing lists or forums. See Section 1.6.1, “MySQL Mailing Lists”,
and Section 1.6.2, “MySQL Community Support at the MySQL Forums”. If you have suggestions concerning
additions or corrections to the manual itself, please send them to
the documentation team at <docs@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, Jon Stephens, Martin MC Brown, and Peter Lavin. For the many other contributors, see Appendix G, Credits.
The copyright to this manual is owned by the Swedish company MySQL AB. MySQL® 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.
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 heremysql>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, SELECT ... INTO
OUTFILE is shorthand for the form of
SELECT statement that has an INTO
OUTFILE clause following other parts of the 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:
RESETreset_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 gccshell>./configure
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 Section 1.4, “Overview of the MySQL Database Management System”.
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.
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 Section 7.1.4, “The MySQL Benchmark Suite”.
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.
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.
MySQL is named after co-founder Monty Widenius's daughter, My.
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.
This section describes some of the important characteristics of the MySQL Database Software. See also Section 1.5, “MySQL Development Roadmap”, for more information about current and upcoming features. In most respects, it applies to all versions of MySQL. For information about features as they are introduced into MySQL on a series-specific basis, see the “In a Nutshell” section of the appropriate Manual:
MySQL 4.0 and 4.1: MySQL 4.0 in a Nutshell, and MySQL 4.1 in a Nutshell.
MySQL 5.0: MySQL 5.0 in a Nutshell.
MySQL 5.1: MySQL 5.1 in a Nutshell.
Internals and Portability:
Written in C and C++.
Tested with a broad range of different compilers.
Works on many different platforms. See Section 2.4.2, “Operating Systems Supported by MySQL Community Server”.
Uses GNU Automake, Autoconf, and Libtool for portability.
The MySQL Server design is multi-layered with independent modules.
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 provide an SQL interface for 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 Chapter 10, Data Types.
Fixed-length and variable-length records.
Statements and Functions:
Full operator and function support in the
SELECT list and WHERE
clause 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 statement can be
used to retrieve information about databases, storage engines,
tables, and indexes. MySQL 5.0 adds support for the
INFORMATION_SCHEMA database, implemented
according to standard SQL.
The EXPLAIN statement 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
Section 8.3, “Reserved Words”.
You can refer to tables from different databases in the same statement.
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 MySQL Server using several protocols:
Clients can connect using TCP/IP sockets on any platform.
On Windows systems in the NT family (NT, 2000, XP, 2003,
or Vista), clients can connect using named pipes if the
server is started with the
--enable-named-pipe option. 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.
On Unix systems, clients can connect using Unix domain socket files.
MySQL client programs can be written in many languages. A client library written in C is available for clients written in C or C++, or for any language that provides C bindings.
APIs for C, C++, Eiffel, Java, Perl, PHP, Python, Ruby, and Tcl are available, allowing MySQL clients to be written in many languages. See Chapter 23, APIs and Libraries.
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 Chapter 24, 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 Chapter 24, 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 Chapter 24, Connectors.
Localization:
The server can provide error messages to clients in many languages. See Section 9.3, “Setting the Error Message Language”.
Full support for several different character sets, including
latin1 (cp1252), german,
big5, ujis, and more.
For example, the Scandinavian characters
“å”,
“ä” and
“ö” 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.
Sorting and comparisons are done according to the chosen
character set and collation (using latin1
and 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.
As of MySQL 4.1, the server time zone can be changed dynamically, and individual clients can specify their own time zone. Section 9.6, “MySQL Server Time Zone Support”.
MySQL Enterprise For assistance in getting optimal performance from your MySQL server subscribe to MySQL Enterprise. For more information see http://www.mysql.com/products/enterprise/.
Clients and Tools:
MySQL AB provides several client and utility programs. These include both command-line programs such as mysqldump and mysqladmin, and graphical programs such as MySQL Administrator and MySQL Query Browser.
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 Chapter 4, MySQL Programs.
MySQL programs can be invoked with the --help
or -? option to obtain online assistance.
This section describes the general 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 currently is 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 Section 2.4.17, “Upgrading MySQL”.
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 |
| 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 |
| Triggers | 5.0 and 5.1 |
| Event scheduler | 5.1 |
| Partitioning | 5.1 |
| Pluggable storage engine API | 5.1 |
| Plugin API | 5.1 |
| Row-based replication | 5.1 |
| Server log tables | 5.1 |
| Foreign keys | 6.x (implemented in 3.23 for InnoDB) |
The following features are implemented in MySQL 5.0.
BIT Data
Type: Can be used to store numbers in binary
notation. See Section 10.1.1, “Overview of Numeric Types”.
Cursors: Elementary support
for server-side cursors. For information about using cursors
within stored routines, see Section 18.2.9, “Cursors”. For
information about using cursors from within the C API, see
Section 23.2.7.3, “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
Chapter 21, INFORMATION_SCHEMA Tables.
Instance Manager: Can be used to start and stop the MySQL Server, even from a remote host. See Section 4.6.9, “mysqlmanager — The MySQL 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 Chapter 22, Precision Math.
Storage Engines: Storage
engines added in MySQL 5.0 include
ARCHIVE and FEDERATED.
See Section 13.8, “The ARCHIVE Storage Engine”, and
Section 13.7, “The FEDERATED Storage Engine”.
Stored Routines: Support for named stored procedures and stored functions was implemented in MySQL 5.0. See Chapter 18, Stored Procedures and Functions.
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 Section 5.1.6, “SQL Modes”.
Triggers: MySQL 5.0 added limited support for triggers. See Chapter 19, Triggers, and Section 1.8.5.3, “Stored Routines and Triggers”.
VARCHAR Data
Type: The effective maximum length of a
VARCHAR column was increased to 65,535
bytes, and stripping of trailing whitespace was eliminated.
(The actual maximum length of a VARCHAR
is determined by the maximum row size and the character set
you use. The maximum effective column
length is subject to a row size of 65,535 bytes, which is
shared among all columns.) See
Section 10.4, “String Types”.
Views: MySQL 5.0 added support for named, updatable views. See Chapter 20, Views, and Section 1.8.5.5, “Views”.
XA Transactions: See Section 12.4.7, “XA Transactions”.
MySQL Enterprise For assistance in maximizing your usage of the many new features of MySQL, subscribe to MySQL Enterprise. For more information see http://www.mysql.com/products/enterprise/advisors.html.
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!N is the number of tables to
be joined. The greedy optimizer reduces this to
N!/(D-1)!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
Section 7.2.6, “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
implies these other conditions
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
Section 7.2.9, “Nested Join Optimization”, and
Section 7.2.10, “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.
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 Section 12.3.2, “EXPLAIN Syntax”, for more information.
(See Chapter 16, MySQL Cluster.)
For those wishing to take a look at the bleeding edge of MySQL development, we make our BitKeeper repository for MySQL publicly available. See Section 2.4.15.3, “Installing from the Development Source Tree”.
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.
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 Section 1.7, “How to Report Bugs or Problems”.
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@yahoogroups.com>
A French mailing list.
A Korean mailing list. To subscribe, email subscribe
mysql your@email.address to this list.
<mysql-de-request@lists.4t2.com>
A German mailing list. To subscribe, email subscribe
mysql-de your@email.address to this list. You can
find information about this mailing list at
http://www.4t2.com/mysql/.
<mysql-br-request@listas.linkway.com.br>
A Portuguese mailing list. To subscribe, email
subscribe mysql-br your@email.address to
this list.
A Spanish mailing list. To subscribe, email subscribe
mysql your@email.address to this list.
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.
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
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/).
MySQL AB offers technical support in the form of MySQL Enterprise. For organizations that rely on the MySQL DBMS for business-critical production applications, MySQL Enterprise is a commercial subscription offering which includes:
MySQL Enterprise Server
MySQL Enterprise Monitor
Monthly Rapid Updates and Quarterly Service Packs
MySQL Knowledge Base
24x7 Technical and Consultative Support
MySQL Enterprise is available in multiple tiers, giving you the flexibility to choose the level of service that best matches your needs. For more information see MySQL Enterprise.
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@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 Section B.1, “Problems and Common Errors”.
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 Section 1.6.1, “MySQL 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@mysql.com>.
To discuss problems with other users, you can use one of the MySQL mailing lists. Section 1.6.1, “MySQL 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 MySQL Internals: Porting.
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 the MyODBC section of Chapter 24, Connectors.
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 MySQL Internals: Porting.
If a database table is related to the problem, include the
output from the SHOW CREATE TABLE
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.
db_name.tbl_name
The SQL mode in effect when the problem occurred can be
significant, so please report the value of the
sql_mode system variable. For stored
procedure, stored function, and trigger objects, the relevant
sql_mode value is the one in effect when the
object was created. For a stored procedure or function, the
SHOW CREATE PROCEDURE or SHOW CREATE
FUNCTION statement shows the relevant SQL mode, or you
can query INFORMATION_SCHEMA for the
information:
SELECT ROUTINE_SCHEMA, ROUTINE_NAME, SQL_MODE FROM INFORMATION_SCHEMA.ROUTINES;
For triggers, you can use this statement:
SELECT EVENT_OBJECT_SCHEMA, EVENT_OBJECT_TABLE, TRIGGER_NAME, SQL_MODE FROM INFORMATION_SCHEMA.TRIGGERS;
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
for each table
that is involved. The more information you provide about your
situation, the more likely it is that someone can help you.
tbl_name
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
Chapter 5, MySQL Server 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
Section 8.2.2, “Identifier 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 Section 5.2.1, “The 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
Section B.1.1, “How to Determine What Is Causing a Problem”.
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 Section 2.4.3, “Choosing Which MySQL Distribution to Install”.
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).
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 Section 12.2.3, “HANDLER Syntax”.
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
Chapter 16, MySQL Cluster.
XML support is to be implemented in a future version of the database server.
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.
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="
option. You can also change the mode at runtime by setting the
mode_value"sql_mode system variable with a SET
[SESSION|GLOBAL]
sql_mode='
statement.
mode_value'
For more information on setting the SQL mode, see Section 5.1.6, “SQL Modes”.
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
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 Section 5.1.2, “Command Options”, and Section 1.8.2, “Selecting SQL Modes”.
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 Section 8.2.2, “Identifier 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
Section 12.1.3, “CREATE DATABASE Syntax”,
Section 12.1.6, “DROP DATABASE Syntax”, and
Section 12.1.1, “ALTER DATABASE Syntax”.
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
Section 12.5.3, “SET Syntax”.
The SHOW statement. See
Section 12.5.4, “SHOW Syntax”. 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
Chapter 21, INFORMATION_SCHEMA Tables.
Use of LOAD DATA INFILE. In many
cases, this syntax is compatible with Oracle's
LOAD DATA INFILE. See
Section 12.2.5, “LOAD DATA INFILE Syntax”.
Use of RENAME TABLE. See
Section 12.1.9, “RENAME TABLE Syntax”.
Use of REPLACE instead of
DELETE plus
INSERT. See
Section 12.2.6, “REPLACE Syntax”.
Use of CHANGE
,
col_nameDROP
, or
col_nameDROP INDEX, IGNORE
or RENAME in ALTER
TABLE statements. Use of multiple
ADD, ALTER,
DROP, or CHANGE
clauses in an ALTER TABLE statement.
See Section 12.1.2, “ALTER TABLE Syntax”.
Use of index names, indexes on a prefix of a column, and
use of INDEX or
KEY in CREATE
TABLE statements. See
Section 12.1.5, “CREATE TABLE Syntax”.
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
syntax.
tbl_name
SET col_name = ...
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 Section 12.2.7, “SELECT Syntax”.
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
Section 11.11, “Functions and Modifiers for Use with GROUP BY Clauses”.
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
where
value_list)value_list has more than one
element.
String comparisons are case-insensitive by default, with
sort ordering determined by the 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 %
MMOD(.
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
Section 11.10.3, “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
Section 11.11, “Functions and Modifiers for Use with GROUP BY Clauses”.
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.
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 Section B.1.8, “Known Issues in MySQL”.
In some cases, CHAR columns are silently
converted to VARCHAR columns when you
define a table or alter its structure. (This no longer
occurs as of MySQL 5.0.3). See
Section 12.1.5.1, “Silent Column Specification 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 Section 12.5.1.5, “REVOKE Syntax”.
The CAST() function does not
support cast to REAL or
BIGINT. See
Section 11.9, “Cast Functions and Operators”.
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.
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
Section 12.2.4.1, “INSERT ... SELECT Syntax”. 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 Section 18.2.7.3, “SELECT ... INTO Statement”.
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
Chapter 13, Storage Engines. For information about
InnoDB differences from standard SQL with
regard to treatment of transaction errors, see
Section 13.2.15, “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.
MySQL Enterprise For expert advice on choosing and tuning storage engines, subscribe to the MySQL Enterprise Monitor. For more information see http://www.mysql.com/products/enterprise/advisors.html.
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
Section 7.3.3, “Concurrent Inserts”, and
Section 12.2.4.2, “INSERT DELAYED Syntax”.
“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:
Use LOCK TABLES to lock all the
tables you want to access.
Test the conditions that must be true before performing the update.
Update if the conditions are satisfied.
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 Section 11.10.3, “Information Functions”, and
Section 23.2.3.37, “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;
Stored procedures and functions are implemented beginning with MySQL 5.0. See Chapter 18, Stored Procedures and Functions.
Basic trigger functionality is implemented beginning with MySQL 5.0.2, with further development planned for MySQL 5.1. See Chapter 19, Triggers.
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
Section 13.2.6.4, “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 Section 7.3.3, “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
Section 12.2.1, “DELETE Syntax”.
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
Section 13.2.6.4, “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 Section 12.2.7.1, “JOIN Syntax”, and
Section 3.6.6, “Using Foreign Keys”.
The FOREIGN KEY syntax without ON
DELETE ... is often used by ODBC applications to
produce automatic WHERE clauses.
Views (including updatable views) are implemented beginning with MySQL Server 5.0.1. See Chapter 20, 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 Section 5.4, “The MySQL Access 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.”
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 Section 8.5, “Comment Syntax”.
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 \| mysqldb_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 Section 4.8.2, “replace — A String-Replacement Utility”.
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 Section 5.1.6, “SQL Modes”.
MySQL Enterprise To be alerted when there is no form of server-enforced data integrity, subscribe to the MySQL Enterprise Monitor. For more information see http://www.mysql.com/products/enterprise/advisors.html.
The following sections describe how MySQL Server handles different types of constraints.
Normally, errors occurs for data-change statements (such as
INSERT or UPDATE) that
would violate primary-key, unique-key, or foreign-key
constraints. 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.
MySQL supports an IGNORE keyword for
INSERT, UPDATE, and so
forth. If you use it, MySQL ignores primary-key or unique-key
violations and continues processing with the next row. See the
section for the statement that you are using
(Section 12.2.4, “INSERT Syntax”, Section 12.2.10, “UPDATE Syntax”, and so
forth).
You can get information about the number of rows actually
inserted or updated with the
mysql_info() C API function.
You can also use the SHOW WARNINGS
statement. See Section 23.2.3.35, “mysql_info()”, and
Section 12.5.4.28, “SHOW WARNINGS Syntax”.
Currently, only InnoDB tables support
foreign keys. See
Section 13.2.6.4, “FOREIGN KEY Constraints”. We plan to
add foreign key support by other storage engines in a future
MySQL release. See Section 1.5, “MySQL Development Roadmap”.
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. Section 5.1.6, “SQL Modes”.
This section describes the default (forgiving) behavior of MySQL, as well as the 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 are handled as described in
Section 1.8.6.3, “ENUM and SET Constraints”.
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
Section 10.1.4, “Data Type Default Values”.
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 the statement has begun executing. We can't just roll back if we encounter a problem after updating a few rows, because the storage engine may not support rollback. The option of terminating the statement is not that good; in this case, the update would be “half done,” which is probably the worst possible scenario. In this case, it's better to “do the best you can” and then continue as if nothing happened.
In MySQL 5.0.2 and up, you can select stricter treatment of
input values by using the
STRICT_TRANS_TABLES or
STRICT_ALL_TABLES SQL modes:
SET sql_mode = 'STRICT_TRANS_TABLES'; SET sql_mode = 'STRICT_ALL_TABLES';
STRICT_TRANS_TABLES enables strict mode for
transactional storage engines, and also to some extent for
non-transactional engines. It works like this:
For transactional storage engines, bad data values occurring anywhere in a statement cause the statement to abort and roll back.
For non-transactional storage engines, a statement aborts
if the error occurs in the first row to be inserted or
updated. (When the error occurs in the first row, the
statement can be aborted to leave the table unchanged,
just as for a transactional table.) Errors in rows after
the first do not abort the statement, because the table
has already been changed by the first row. Instead, bad
data values are adjusted and result in warnings rather
than errors. In other words, with
STRICT_TRANS_TABLES, a wrong value
causes MySQL to roll back all updates done so far, if that
can be done without changing the table. But once the table
has been changed, further errors result in adjustments and
warnings.
For even stricter checking, enable
STRICT_ALL_TABLES. This is the same as
STRICT_TRANS_TABLES except that for
non-transactional storage engines, errors abort the statement
even for bad data in rows following the first row. This means
that if an error occurs partway through a multiple-row insert
or update for a non-transactional table, a partial update
results. Earlier rows are inserted or updated, but those from
the point of the error on are not. To avoid this for
non-transactional tables, either use single-row statements or
else use STRICT_TRANS_TABLES if conversion
warnings rather than errors are acceptable. To avoid problems
in the first place, do not use MySQL to check column content.
It is safest (and often faster) to let the application ensure
that it passes only legal values to the database.
With either of the strict mode options, you can cause errors
to be treated as warnings by using INSERT
IGNORE or UPDATE IGNORE rather
than INSERT or UPDATE
without IGNORE.
ENUM and SET columns
provide an efficient way to define columns that can contain
only a given set of values. See Section 10.4.4, “The ENUM Type”, and
Section 10.4.5, “The SET Type”. However, before MySQL 5.0.2,
ENUM and SET columns do
not provide true constraints on entry of invalid data:
ENUM columns always have a default
value. If you specify no default value, then it is
NULL for columns that can have
NULL, otherwise it is the first
enumeration value in the column definition.
If you insert an incorrect value into an
ENUM column or if you force a value
into an ENUM column with
IGNORE, it is set to the reserved
enumeration value of 0, which is
displayed as an empty string in string context.
If you insert an incorrect value into a
SET column, the incorrect value is
ignored. For example, if the column can contain the values
'a', 'b', and
'c', an attempt to assign
'a,x,b,y' results in a value of
'a,b'.
As of MySQL 5.0.2, you can configure the server to use strict
SQL mode. See Section 5.1.6, “SQL Modes”. With strict
mode enabled, the definition of a ENUM or
SET column does act as a constraint on
values entered into the column. An error occurs for values
that do not satisfy these conditions:
An ENUM value must be one of those
listed in the column definition, or the internal numeric
equivalent thereof. The value cannot be the error value
(that is, 0 or the empty string). For a column defined as
ENUM('a','b','c'), values such as
'', 'd', or
'ax' are illegal and are rejected.
A SET value must be the empty string or
a value consisting only of the values listed in the column
definition separated by commas. For a column defined as
SET('a','b','c'), values such as
'd' or 'a,b,c,d' are
illegal and are rejected.
Errors for invalid values can be suppressed in strict mode if
you use INSERT IGNORE or UPDATE
IGNORE. In this case, a warning is generated rather
than an error. For ENUM, the value is
inserted as the error member (0). For
SET, the value is inserted as given except
that any invalid substrings are deleted. For example,
'a,x,b,y' results in a value of
'a,b'.