Thanks everybody for the inputs!!

Ann, You are right: space is no longer an issue these days... It's just that I always keep asking whether is "the best" or the "correct" way to do it...

I'm not an expert in the matter, but I guess that having a field which in most of cases will be null, is against normalization rules. Isn't it?

Anyway, I will put it on the main invoices table, and stop messing with this kind of questions!!

Thanks again!! :)