Re: Foreign keys

"Kira Yamato" <kirakun_at_earthlink.net> wrote in message news:2008011502240916807-kirakun_at_earthlinknet...
> On 2008-01-14 21:18:57 -0500, "Evan Keel" <evankeel_at_sbcglobal.net> said:
>
> > Always a physical issue. Never a theory issue.Agree?
>
> Foreign keys are functional dependencies across two relations.
>
> More specifically, let
> R1(K1, A1, B1)
> be a relation with attribute sets K1, A1 and B1 where K1 is R1's
> primary key and B1 is a foreign key to the relation
> R2(K2, A2)
> where K2 is R2's primary key and A2 is the set of its remaining
attributes.
>
> Then the foreign key B1 represents the functional dependency
> B1 --> A2,
> which is the functional dependency across two relation I mentioned in
> the first sentence.
>
> Furthermore, through transitivity by the functional dependency K1 -->
> B1, the foreign key also represents the inter-relational functional
> dependency
> K1 --> A2.
>
> Am I correct to say this?
>
> --
>
> -kira
>
Not reading the whole thread (big mistake on my part), but how about the instance where a foreign key can be null?. No functional dependecy but legit.

Evan Received on Thu Jan 17 2008 - 17:08:07 CET