Chapter 10 MySQL Differences from Standard SQL

MySQL Server doesn't support the SELECT ... INTO TABLE Sybase SQL extension. Instead, MySQL Server supports the INSERT INTO ... SELECT standard SQL syntax, which is basically the same thing. See INSERT ... SELECT Syntax. For example:

INSERT INTO tbl_temp2 (fld_id)
    SELECT tbl_temp1.fld_order_id
    FROM tbl_temp1 WHERE tbl_temp1.fld_order_id > 100;

Alternatively, you can use SELECT ... INTO OUTFILE or CREATE TABLE ... SELECT.

You can use SELECT ... INTO with user-defined variables. The same syntax can also be used inside stored routines using cursors and local variables. See SELECT ... INTO Syntax.

If you access a column from the table to be updated in an expression, UPDATE uses the current value of the column. The second assignment in the following statement sets col2 to the current (updated) col1 value, not the original col1 value. The result is that col1 and col2 have the same value. This behavior differs from standard SQL.

UPDATE t1 SET col1 = col1 + 1, col2 = col1;

MySQL's implementation of foreign keys differs from the SQL standard in the following key respects:

For information about how the InnoDB storage engine handles foreign keys, see InnoDB and FOREIGN KEY Constraints.

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 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 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 3.23.3 and higher. Therefore, credit--1 is safe to use.

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 replace — A String-Replacement Utility.