Data extraction from 2 tables
It is not nice - sort of anti-join

http://sqlfiddle.com/#!9/cb0124/3

SELECT a1.x, b.z
FROM (
        (SELECT DISTINCT x FROM a) a1,
        b
     ) LEFT JOIN a a2 ON (b.z = a2.y AND a2.x=a1.x)
WHERE a2.x IS NULL;