13.4 '--' as the Start of a Comment

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 enable MySQL-specific SQL to be embedded in the comment; see Comments.

MySQL Server also uses # as the start comment character. This is nonstandard.

Standard SQL also uses “--” as a start-comment sequence. MySQL Server supports a variant of the -- comment style; the -- start-comment sequence is accepted as such, but must be followed by a whitespace character such as a space or newline. The space is intended to prevent problems with generated SQL queries that use constructs such as the following, which updates the balance to reflect a charge:

UPDATE account SET balance=balance-charge
WHERE account_id=user_id

Consider what happens when charge has a negative value such as -1, which might be the case when an amount is credited to the account. In this case, the generated statement looks like this:

UPDATE account SET balance=balance--1
WHERE account_id=5752;

balance--1 is valid standard SQL, but -- is interpreted as the start of a comment, and part of the expression is discarded. The result is a statement that has a completely different meaning than intended:

UPDATE account SET balance=balance
WHERE account_id=5752;

This statement produces no change in value at all. To keep this from happening, MySQL requires a whitespace character following the -- for it to be recognized as a start-comment sequence in MySQL Server, so that an expression such as balance--1 is always safe to use.