Re: Foreign keys

"Bob Badour" <bbadour_at_pei.sympatico.ca> wrote in message news:478f81dc$0$4062$9a566e8b_at_news.aliant.net...
> Evan Keel wrote:
>
> > "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
>
> The legitimacy is predicated on the legitimacy of NULL in the first place.

Absolutely agree.

Evan Received on Thu Jan 17 2008 - 17:28:59 CET