> There's no way to tell "I know, it's empty. No, it's not unknown
> because of that, it's empty and so it's different with something that
> is not empty"

If you want to say "for this entity this attribute is not applicable",
then your table is not normalized enough. But if "empty" is part of the
attribute's domain, then make it explicit and put some value to the
attribute (-1 for example).

I think that confusion comes from the programming language, where a null
pointer/reference compared to NULL constant gives true.

Roman