MySQL Server supports three comment styles:
#character to the end of the line.
--sequence to the end of the line. In MySQL, the
--(double-dash) comment style requires the second dash to be followed by at least one whitespace or control character (such as a space, tab, newline, and so on). This syntax differs slightly from standard SQL comment syntax, as discussed in Section 188.8.131.52, “'--' as the Start of a Comment”.
/*sequence to the following
*/sequence, as in the C programming language. This syntax enables a comment to extend over multiple lines because the beginning and closing sequences need not be on the same line.
The following example demonstrates all three comment styles:
mysql> SELECT 1+1; # This comment continues to the end of line mysql> SELECT 1+1; -- This comment continues to the end of line mysql> SELECT 1 /* this is an in-line comment */ + 1; mysql> SELECT 1+ /* this is a multiple-line comment */ 1;
Nested comments are not supported, are deprecated, and will be removed in a future MySQL release. (Under some conditions, nested comments might be permitted, but usually are not, and users should avoid them.)
MySQL Server supports certain variants of C-style comments. These enable you to 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
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
KEY_BLOCK_SIZE keyword in the
following comment is executed only by servers from MySQL 5.1.10 or
CREATE TABLE t1(a INT, KEY (a)) /*!50110 KEY_BLOCK_SIZE=1024 */;
The comment syntax just described applies to how the mysqld server parses SQL statements. The mysql client program also performs some parsing of statements before sending them to the server. (It does this to determine statement boundaries within a multiple-statement input line.) For information about differences between the server and mysql client parsers, see Section 184.108.40.206, “mysql Client Tips”.
/*!12345 ... */ format are not
stored on the server. If this format is used to comment stored
programs, the comments are not retained in the program body.
Another variant of C-style comment syntax is used to specify
optimizer hints. Hint comments include a
character following the
/* comment opening
SELECT /*+ BKA(t1) */ FROM ... ;
For more information, see Section 8.9.3, “Optimizer Hints”.
The use of short-form mysql commands such as
\C within multiple-line
*/ comments is not supported. Short-form commands do
work within single-line
/*! ... */ version
comments, as do
/*+ ... */ optimizer-hint
comments, which are stored in object definitions. If there is a
concern that optimizer-hint comments may be stored in object
definitions so that dump files when reloaded with
mysql would result in execution of such
commands, either invoke mysql with the
--binary-mode option or use a reload
client other than mysql.