> >
> > We used not to be able to have FKs with duplicate NULLs. Now we
> can. this
> > leads to table structures being able to consolidate data from several
> > sources leaving certain FKs null while the critical one for this
> > relationship is filled. e.g. lets imagine a contacts list for 3 separate
> > tables, client, contractors and suppliers.
>
> I'm not sure I understand the design, but is sounds like bad
> normalization ... Consolidation is rarely a good thing in relational
> theory.
>

OK - use of the word relation is misleading. And maybe consolidation is not
a good thing.
Answer me this then batman...
It used to be illegal to have duplicate null values in an FK constraint
field. I always understood that and it kept me on the straight and narrow.
Now you can have duplicate nulls - why was this permitted?
or was this all so long ago I've forgotten what I was doing back then. Was
it, fact, "you can't have an FK unless it's declared not null" and now you
can have a nullable FK field. Something has changed in this regard.
Alan