http://dev.mysql.com/doc/refman/5.0/en/row-subqueries.html

The discussion to this point has been of scalar or column subqueries; that is, subqueries that return a single value or a column of values. A row subquery is a subquery variant that returns a single row and can thus return more than one column value. Here are two examples:

SELECT * FROM t1 WHERE (1,2) = (SELECT column1, column2 FROM t2);
SELECT * FROM t1 WHERE ROW(1,2) = (SELECT column1, column2 FROM t2);