1.9.5.7. '--' 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 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”.
