Re: Constraints and Functional Dependencies
Date: Fri, 02 Mar 2007 19:34:01 GMT
Message-ID: <J__Fh.23$4u5.3_at_trndny09>
"Marshall" <marshall.spight_at_gmail.com> wrote in message
news:1172684041.794087.32540_at_p10g2000cwp.googlegroups.com...
> On Feb 28, 8:00 am, "Walt" <wami..._at_verizon.net> wrote:
> > "Marshall" <marshall.spi..._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?
>
> That sounds exactly right to me.
>
> Although, again, in practice it is likely that b will be a candidate
> key.
> >
> Marshall
>
It seems to me that one can discover meaningful joins between data sets that, prior to discovery, are seen as unrelated. In order to avoid hijacking this discussion, I'm going to start a new one. Received on Fri Mar 02 2007 - 20:34:01 CET