The MySQL server supports three comment styles:
From a ‘#’ character to the end of the line.
From a ‘-- ’ sequence to the end of the line. This style is supported as of MySQL 3.23.3. Note that the ‘-- ’ (double-dash) comment style requires the second dash to be followed by at least one space (or by a control character such as a newline). This syntax differs slightly from standard SQL comment syntax, as discussed in the section called “‘--’ as the Start of a Comment”.
From a ‘/*’ sequence to the following ‘*/’ sequence. The closing sequence need not be on the same line, so this syntax allows a comment to extend over multiple lines.
The following example demonstrates all three 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;
The comment syntax just described applies to how the mysqld server parses SQL statements. The mysql client program also performs some parsing of statements before sending them to the server. (For example, it does this to determine statement boundaries within a multiple-statement input line.) However, there are some limitations on the way that mysql parses /* ... */ comments:
A semicolon within the comment is taken to indicate the end of the current SQL statement and anything following it to indicate the beginning of the next statement. This problem was fixed in MySQL 4.0.13.
A single quote, double quote, or backtick character is taken to indicate the beginning of a quoted string or identifier, even within a comment. If the quote is not matched by a second quote within the comment, the parser doesn't realize 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 `>. This problem was fixed in MySQL 4.1.1.
For affected versions of MySQL, these limitations apply both when you run mysql interactively and when you put commands in a file and use mysql in batch mode to process the file with mysql < file_name.