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,
as described in Section 9.6, “Comments”.
Standard SQL uses “--” as a
start-comment sequence. MySQL Server uses #
as the start comment character. MySQL Server 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 valid 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 permitting comments to start with
-- can have serious consequences.
Using our implementation requires a space following the
-- for it to be recognized as a
start-comment sequence in MySQL Server. Therefore,
credit--1 is safe to use.