sql 組合查詢

SELECT A.a,
CASE WHEN EXISTS
(
   select B.b from A,B where B.a =A.a
) THEN 'true' ELSE 'false'
END,
CASE WHEN EXISTS
(
   select C.c from A,C where C.a =A.a
) THEN 'true' ELSE 'false'

END
FROM A