MS-SQL만 만지다가 firebird를 사용하게 되었습니다.

firebird는 from절에 서브쿼리가 안들어 가던데... 당췌 쿼리문을 못 만들겠네요.. ㅜ.ㅜ;

table1
--------------------------------
a              b              c              d
--------------------------------
가             2             12             Y
나             3             0              N
다             6             2              N
가            90             87            Y
나            12              5             Y

select a, sum(b), sum(c) from (
   select a, sum(b) as b, 0 as c from table1 where d='Y'
   group by a, d
   union all
   select a, 0 as b, sum(c) as c from table1 where d='N'
   group by a, d
) z
group by a

이것좀 firebird 쿼리로 바꿔주세요...  부탁드립니다.

도와주세요~~~~