想对查询出来的结果集再次操作，就可以把查询出来的结果集当成一个临时表
对临时表t1操作
语法：select sum(a), b from
( select a, b ,c
from table1
join table2
on table1.a = table2.a
where table1.a='1') t
group by b
复杂一点，对两个临时结果集t1,t2操作
select sum(a), b from
( select a, b ,c
from table1
join table2
on table1.a = table2.a
where table1.a='1') t1
join t2
( select a, b ,c
from table1
join table2
on table1.a = table2.a
where table1.a='1') t2
on t1.a=t2.a
group by b
order by b
需要注意一点的是order by 语句是不能放在内层查询语句的，只能放在外面