6.1.6 Comment Syntax

The MySQL server supports the # to end of line, -- to end of line and /* in-line or multiple-line */ comment styles:

mysql> SELECT 1+1;     # This comment continues to the end of line
mysql> SELECT 1+1;     -- This comment continues to the end of line
mysql> SELECT 1 /* this is an in-line comment */ + 1;
mysql> SELECT 1+
/*
this is a
multiple-line comment
*/
1;

Note that the -- (double-dash) comment style requires you to have at least one space after the second dash!

Although the server understands the comment syntax just described, there are some limitations on the way that the mysql client parses /* ... */ comments:

• Single-quote and double-quote characters are taken to indicate the beginning of a quoted string, even within a comment. If the quote is not matched by a second quote within the comment, the parser doesn't realise the comment has ended. If you are running mysql interactively, you can tell that it has gotten confused like this because the prompt changes from mysql> to '> or ">.
• A semicolon is taken to indicate the end of the current SQL statement and anything following it to indicate the beginning of the next statement.

These limitations apply both when you run mysql interactively and when you put commands in a file and tell mysql to read its input from that file with mysql < some-file.

MySQL supports the `--' SQL-99 comment style only if the second dash is followed by a space. See section 1.8.4.7 `--' as the Start of a Comment.

User Comments

YOu may also embed non-ANSI MYSQL commands
or commands specifiec to a particular version in the
comment syntax using the /*!xxxx */ syntax See
section:
1.7.3 MySQL Extensions to ANSI SQL92

In version 3.23, a semicolon in a multiline
comment will cause an error. Including the GPL
header in a comment, for instance, will generate
lots of errors. (Legalese uses lots of semicolons).

Some tiny gotchas-- nothing serious. Using two dashes ('--') inside a regular comment block (/* ... */) __MAY__ confuse the parser and in which case you will get strange errors. Also using single apostrophe's (') can wig it out too. Not sure why this happens.

/*
this is a good comment
*/

/* this is a bad comment-- it has two dashes
and __MIGHT__ wig out the parser
(for some reason it doesn't always happen, haven't
figure out the how/when part yet)
*/

My guess is the Parser "Wigs Out" when you include the double dash on the same line as the */, so that it looks like this:

/* Start a Comment
-- Here's a nested comment */


Now, the parser saw the /* and the --, but because of the --, it didn't see the */ and so it thinks the original /* comment is still active. It closed the -- comment on the carriage return.

The parser should either NOT recognize nested comments, or be adjusted to allow for closing one comment while "technically" inside another.

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