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:
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 file names 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 file names (such as most Unix systems). See Section 9.2.2, "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
.frm files to which the table corresponds. (Nevertheless, it
is preferable to use
ALTER TABLE ... RENAME and let the
server rename the files.)
General language syntax
By default, strings can be enclosed by either "
just by "
'". (If the
ANSI_QUOTES SQL mode is enabled, strings can be enclosed
only by "
'" and the server interprets strings enclosed
is the escape character in strings.
In SQL statements, you can access tables from different databases with
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
SELECT to obtain a description of how tables are processed by the query
SHOW statement. See Section
SHOW Syntax". The information produced by many of
statements can be obtained in more standard fashion by using
SELECT to query
INFORMATION_SCHEMA. See Chapter
DROP , or
statements. Use of multiple
CHANGE clauses in an
ALTER TABLE statement. See Section
ALTER TABLE Syntax".
NOT EXISTS with
The capability of dropping multiple tables with a single
DROP TABLE statement.
INSERT INTO syntax.
Options such as
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
12.17, "Functions and Modifiers for Use with
You can specify
GROUP BY, not just with
The ability to set variables in a statement with the
:= assignment operator. See Section
9.4, "User-Defined Variables".
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
&& operators to mean logical OR and AND, as in the C
programming language. In MySQL Server,
OR are synonyms, as are
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 is easy to
convert use of the
operator to MySQL Server.
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
(cp1252 West European) by default. If you don't like this, you should declare your columns
BINARY attribute or use the
cast, which causes comparisons to be done using the underlying character code values rather
than a lexical ordering.
SELECT col1=1 AND col2=2 FROM my_table;
LIKE is permitted on numeric values.
TRIM() to trim substrings. Standard SQL supports removal of
single characters only.