Thanks Alan & Svein for both answers !!!
May I ask two more questions?

1) I thought that the SUM function didn't use the NULL values. Actually, what I
was told is that none of the aggregate functions uses NULL values. So I never
check for nulls when I use SUM(). Am I wrong?

2) Do you think this aproach would be faster than having separated selects?

Thanks again !!

-sergio