求助一较复杂SQL语句

Question

select '200505' time,(select sum(num) from tbl_200504 where time<='200504')-(select sum(num) from tbl_200505 where time<='200504') num from dual;

Answer 1

select a.sum1-b.sum2 from
(select sum(num) sum1 from tbl_200504 where time<=to_date('200504','yyyymm)) a,
(select sum(num) sum2 from tbl_200505 where time<to_date('200505','yyyymm)) b;

Answer 2

select (select max(time) from tbl_200505) as thismonth,
sum(a.num)-sum(b.num) from tbl_200505 a,tbl_200504 b 
where a.time<=(select max(time) from tbl_200505) 
and b.time <(select max(time) from tbl_200505)