Re: Basic question?What 's the key if there 's no FD(Functional Dependencies)?

From: David Cressey <dcressey_at_verizon.net>
Date: Thu, 02 Nov 2006 12:48:49 GMT
Message-ID: <ROl2h.10257$gf5.7593@trndny01>

"paul c" <toledobythesea_at_dbms.yuc> wrote in message news:a2c2h.242940$R63.209531_at_pd7urf1no...
> vldm10 wrote:
> > saturnlee_at_yahoo.com wrote:
> >> What 's the key for it? ABC or nothing???
> >
> >
> > ABC is not the key.
> > Example: Let one partricular entity has A,B,C atributes
> > and let these atributes take the following values:
> >
> > A B C
> > -----------------------------
> > 2 4 6
> > 8 4 6
> > 2 4 6
> >
> > ( ABC can be the key only in the trivial cases i.e if an entity has
> > the atributes whose values never change)
> >

>
> There are more precise writers than I here but since they haven't
commented:

>

> 1) Are you talking about SQL? If so, I think you should have said so
> because ABC is certainly a candidate key if the above table is a
> relation and no subset of {A,B,C} is stipulated as a key.

>

> 2) I don't know why entities need to be mentioned, either, nor what a
> non-"trivial" entity might possibly be.
>
> p

I think he was referring to "trivial functional dependencies". A key determines any subset of itself, trivially. In current parlance, "well, duh!" Received on Thu Nov 02 2006 - 06:48:49 CST