how to sum conditionally?

Question

I have a following table

Col1   Col2   Col3
A1     1      null
A1     2      null
B1     5      null
B2     6      null
M1     1      M
M2     2      M
M3     3      M
J1     1      J
J2     2      J

I want to sum Col2 based on Col1. The query will be like following,

select Col1, sum (Col2)
group by Col1

However, if Col3 has the same letter, I want sum up Col2 for all Col1. So the result table should be like

Col1    Col2 
A1      3
B1      5
B2      6
M1      6
M2      6
M3      6
J1      3
J2      3

How do I change my query to get above table?

Answer 1

Edit after comment / update to question. I didn't know a clever way, seems like some others have one though.

select * from (
select Col1, SUM(Col2) Col2
from Table
where Col3 is null
group by Col1

union

select mainset.Col1, tmp.Col2
from Table mainset
join 
(
    select Col3, SUM(Col2) Col2
    from Table
    where Col3 is not null
    group by Col3
) tmp on tmp.Col3 = mainset.Col3

where mainset.Col3 is not null
) fullset
order by fullset.Col1