1.8.4.7 `--' as the Start of a Comment

Some other SQL databases use `--' to start comments. MySQL Server has `#' as the start comment character. You can also use the C comment style /* this is a comment */ with MySQL Server. See section 6.1.6 Comment Syntax.

MySQL Server Version 3.23.3 and above support the `--' comment style, provided the comment is followed by a space. This is because this comment style has caused many problems with automatically generated SQL queries that have used something like the following code, where we automatically insert the value of the payment for !payment!:

UPDATE tbl_name SET credit=credit-!payment!

Think about what happens if the value of payment is negative. Because 1--1 is legal in SQL, the consequences of allowing comments to start with `--' are terrible.

Using our implementation of this method of commenting in MySQL Server Version 3.23.3 and up, 1-- This is a comment is actually safe.

Another safe feature is that the mysql command-line client removes all 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 program in a text file that contains `--' comments you should use:

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

instead of the usual:

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

You can also edit the command 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

User Comments

I used mysqldump to make a dump of a single database on server version 3.23.52 and was unable to import that data on a 3.23.43 server. Although this manual entry states that server versions > 3.23.3 recognize the new '--', I discovered that this particular server version chokes on lines that consist of just dashes ('-').

I also couldn't get the 'replace' example to work on my system until I did:

shell> replace "\^--" "#" -- my-file.sql

Which will replace only lines that start with '--'. I hope this helps someone with a similar problem.
p.

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