Union muligheder

Er det muligt at selecte videre på en union a la:
Det inden for parenteserne virker fint men understøtter mysql overhovedet at man selecter videre... ?

SELECT a FROM
(SELECT a FROM table_name WHERE a=10 AND B=1 ORDER BY a LIMIT 10)
UNION
(SELECT a FROM table_name WHERE a=11 AND B=2 ORDER BY a LIMIT 10)
Where a > 4