SQL 查询相加显示的问题。

c1
id  id2  sl
----------------
a1  01    50
a2  02    20

c2
id  id2  sl
----------------
a1  01    20
a1  02    20

组合显示出来
id，id2相同的sl相加

想要的结果
id  id2  sl
----------------
a1  01    70
a2  02    20
a1  02    20

请问SQL语句怎么写？
我用select a.id,a.id2,sum(a.sl+a.sl) from c1 a left join c2 b on a.id=b.id and a.id2=b.id2 group by a.di,a.di2
显示不对。少一行。

---

select
  a.id,
  a.id2,
  sum(a.sl+b.sl) as sl
from
  c1 a
left join
  c2 b
on
  a.id=b.id
and
  a.id2=b.id2
group by
  a.di,a.di2
楼主注意红色部分。

---

SQL code:
select id,id2,sum(sl) as sl
from
(select * from c1
union all
select * from c2
) t
group by id,id2

---

SQL code:
create table c1(id varchar(10), id2 varchar(10) , sl int)
insert into c1 values('a1' , '01' , 50 )
insert into c1