Join A to B via AB where AB may or may not join

I have tables A and B which are joined in a binary relationship via AB.

I need to:

select *
from a, b

But the where cause must use the relationship defined in AB if one exists, otherwise join all B rows to every A row where the relationship doesn't exists.

Given:

A -> (A,B,C,D)
B -> (1,2,3,4)
AB -> (A1,A2,B3)

Should result in:
A1, A2, B3, C1, C2, C3, C4, D1, D2, D3, D4

Can anyone help with this. I know I've done it before, but can't remember how.

Thanks Received on Fri Dec 14 2007 - 12:54:58 CST