一个简单的sql语句

[sjd107]

col1 col2 col3 id
------------------------------
1 1 a a
1 2 b a
1 3 a b

select sum(a.col2),sum(b.col2) from mytab a,mytab b
where a.col3=a and b.col3=b and a.id=b.id
group by a.col1

sum(a.col2) sum(b.col2)
-----------------------------
1 1 2

我要把3也加到a.col2中，因该怎么写？

[beckhambobo]

select col1,sum(decode(col3,'a',col2)),sum(deocde(col3,'b',col2)) from mytab group by col1