> Why you don't want to let field to be null AND _use_ foreign key?

Well... at this point, I'm sure I was WRONG!! But somehow I believed that that
wasn't a "good practice"... I have just one domain "ID" in my databases defined
as not null and I used it to ALL my IDs (PK or FK)...

I'm happy I asked here, because I now realize there's nothing bad in having FK
null !! So I will create a new domain "ID_FK" which allows null and will change
all (ohh, my god!! there's a lot!!) the FK fields to that domain and will use
null values when I have nothing to point...

THANK YOU VERY MUCH TO ALL OF YOU !!

-s

PS: I'm also happy to know that is not that ugly copying the article's
descriptions to the invoice detail !! :))
Sometimes I couldn't sleep thinking if I should implement something similar to
Mark's suggestion... and again I'm happy to learn that most of the people think
is not a good idea to add such a complexity in de data model...