MySQL Reference Manual - 4.0.20

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14.1.8.11 Rewriting Subqueries as Joins for Earlier MySQL Versions

Before MySQL 4.1, only nested queries of the form INSERT ... SELECT ... and REPLACE ... SELECT ... are supported. The IN() construct can be used in other contexts to test membership in a set of values.

It is often possible to rewrite a query without a subquery:

SELECT * FROM t1 WHERE id IN (SELECT id FROM t2);

This can be rewritten as:

SELECT DISTINCT t1.* FROM t1,t2 WHERE t1.id=t2.id;

The queries:

SELECT * FROM t1 WHERE id NOT IN (SELECT id FROM t2);
SELECT * FROM t1 WHERE NOT EXISTS (SELECT id FROM t2 WHERE t1.id=t2.id);

Can be rewritten as:

SELECT table1.* FROM table1 LEFT JOIN table2 ON table1.id=table2.id
                                       WHERE table2.id IS NULL;

A LEFT [OUTER] JOIN can be faster than an equivalent subquery because the server might be able to optimize it better--a fact that is not specific to MySQL Server alone. Prior to SQL-92, outer joins did not exist, so subqueries were the only way to do certain things in those bygone days. Today, MySQL Server and many other modern database systems offer a whole range of outer join types.

For more complicated subqueries, you can often create temporary tables to hold the subquery. In some cases, however, this option will not work. The most frequently encountered of these cases arises with DELETE statements, for which standard SQL does not support joins (except in subqueries). For this situation, there are three options available:

The first option is to upgrade to MySQL 4.1, which does support subqueries in DELETE statements.
The second option is to use a procedural programming language (such as Perl or PHP) to submit a SELECT query to obtain the primary keys for the records to be deleted, and then use these values to construct the DELETE statement (DELETE FROM ... WHERE key_col IN (key1, key2, ...)).
The third option is to use interactive SQL to construct a set of DELETE statements automatically, using the MySQL extension CONCAT() (in lieu of the standard || operator). For example:
SELECT CONCAT('DELETE FROM tab1 WHERE pkid = ', "'", tab1.pkid, "'", ';') FROM tab1, tab2 WHERE tab1.col1 = tab2.col2;
You can place this query in a script file, use the file as input to one instance of the mysql program, and use the program output as input to a second instance of mysql:
shell> mysql --skip-column-names mydb < myscript.sql | mysql mydb

MySQL Server 4.0 supports multiple-table DELETE statements that can be used to efficiently delete rows based on information from one table or even from many tables at the same time. Multiple-table UPDATE statements are also supported as of MySQL 4.0.