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 9.4, “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 of require a following space for ‘--’ to be recognized as a start-comment sequence in MySQL Server 3.23.3 and up, credit--1 is actually safe.

Another safe feature is that the mysql command-line client ignores lines that start with ‘--’.

The following information is relevant only if you are running a MySQL version earlier than 3.23.3:

If you have an SQL script in a text file that contains ‘--’ comments, you should use the replace utility as follows to convert the comments to use ‘#’ characters before executing the script:

shell> replace " --" " #" < text-file-with-funny-comments.sql \
         | mysql db_name

That is safer than executing the script in the usual way:

shell> mysql db_name < text-file-with-funny-comments.sql

You can also edit the script file “in place” to change the ‘--’ comments to ‘#’ comments:

shell> replace " --" " #" -- text-file-with-funny-comments.sql

Change them back with this command:

shell> replace " #" " --" -- text-file-with-funny-comments.sql

See Section 8.17, “replace — A String-Replacement Utility”.