This method is very useful. I always use this when simple queries of linking two tables is not enough.

This method is putting a query inside a query.

Example:

SELECT * FROM table1 LEFT JOIN (SELECT * FROM table2 WHERE field1='id') AS subtable

or joining multiple queries

SELECT * FROM (SELECT * FROM table1 WHERE userid='001') AS subtable1 LEFT JOIN (SELECT * FROM table2 WHERE field1='id') AS subtable2

You can join more than two queries depending on the output you want.

If someone already posted this topic here I'm sorry I don't know.

Starting with MySQL 4.1, all subquery forms and operations that the SQL standard requires are supported, as well as a few features that are MySQL-specific.

Subqueries in MySQL
http://dev.mysql.com/tech-resources/articl...ies_part_1.html

MySQL 5.0 Reference Manual :: 12 SQL Statement Syntax :: 12.2 Data Manipulation Statements :: 12.2.8 Subquery Syntax
http://dev.mysql.com/doc/refman/5.0/en/subqueries.html