Re: Constraints and Functional Dependencies

"Marshall" <marshall.spight_at_gmail.com> wrote in message news:1172367854.472739.82840_at_t69g2000cwt.googlegroups.com...
> On Feb 24, 10:46 am, paul c <toledobythe..._at_oohay.ac> wrote:
> > mAsterdam wrote:
> > > paul c wrote:
> >
> > >> Marshall wrote:
> >
> > >>> ...
> > >>> With such a system, a relation R with attribute a (which I will
> > >>> write as R(a)) having a as a foreign key into S(b) is expressed
> > >>> as follows:
> >
> > >>> forall R(a): exists S(b): a = b
> >
> > >>> So we can express foreign keys this way.
> > >>> ...
> >
> > >> I presume that if S had other attributes besides b, this definition
> > >> would mean that b doesn't need to be a so-called primary key? (That
> > >> would be okay with me.)
> >

Excuse me. b doesn't even need to be a candidate key for S. Your constraint says that there exists at least one S(b) such that a = b, but you haven't said that there exists not more than one S(b). In particular, if there are other attributes in S, b might not be a candidate key, but still satisfy the referential integrity rule you've given.

Or am I misreading something? Received on Wed Feb 28 2007 - 10:00:39 CST