MySQL Server supports three comment styles:
From a “
#” character to the
end of the line.
From a “
-- ” sequence to
the end of the line. This style is supported as of MySQL
3.23.3. 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 126.96.36.199, “'
--' 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:
SELECT 1+1; # This comment continues to the end of linemysql>
SELECT 1+1; -- This comment continues to the end of linemysql>
SELECT 1 /* this is an in-line comment */ + 1;mysql>
this is a
Nested comments are not supported.
MySQL Server supports some 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:
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 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 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.) However, there are some limitations on the way that
/* ... */
A semicolon within the comment is taken to indicate the end of the current SQL statement and anything following it to indicate the beginning of the next statement. This problem was fixed in MySQL 4.0.13.
A single quote, double quote, or backtick character is taken
to indicate the beginning of a quoted string or identifier,
even within a comment. If the quote is not matched by a second
quote within the comment, the parser doesn't realize the
comment has ended. If you are running mysql
interactively, you can tell that it has gotten confused like
this because the prompt changes from
problem was fixed in MySQL 4.1.1.
The use of short-form commands such as
/* ... */ comments is not
Comments in this format,
/*!12345 ... */, are
not stored on the server. If this format is used to comment stored
routines, the comments will not be retained on the server.
For affected versions of MySQL, these limitations apply both when
you run mysql interactively and when you put
commands in a file and use mysql in batch mode
to process the file with mysql <