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On the mysql:
http://dev.mysql.com/doc/mysql/en/update.html

Starting with MySQL 4.0.4, you can also perform UPDATE operations that cover multiple tables:

UPDATE items,month SET items.price=month.priceWHERE items.id=month.id;The example shows an inner join using the comma operator, but multiple-table UPDATE statements can use any type of join allowed in SELECT statements, such as LEFT JOIN.

Note: You cannot use ORDER BY or LIMIT with multiple-table UPDATE.

Before MySQL 4.0.18, you need the UPDATE privilege for all tables used in a multiple-table UPDATE, even if they were not updated. As of MySQL 4.0.18, you need only the SELECT privilege for any columns that are read but not modified.

If you use a multiple-table UPDATE statement involving InnoDB tables for which there are foreign key constraints, the MySQL optimizer might process tables in an order that differs from that of their parent/child relationship. In this case, the statement fails and rolls back. Instead, update a single table and rely on the ON UPDATE capabilities that InnoDB provides to cause the other tables to be modified accordingly.

Currently, you cannot update a table and select from the same table in a subquery.