MySQL 5.7 Release Notes
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 Section 9.6, “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.