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

Brian Selzer wrote:
> "vldm10" <vldm10_at_yahoo.com> wrote in message
> news:1162403098.627006.128180_at_i42g2000cwa.googlegroups.com...
> > 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)
> >

>

I thought in mathematica relation is subset of the product of two or more sets. Or you mean 'relation' from first order logic ? It not contains statements in logic also, its just a formula. Please explain.

--

Tegi



> A key is a subset of the attributes of a relation schema such that the


> projection over that subset onto any relation has the same cardinality of

> that relation.  A candidate key is a key that exhibits the additional

> property that no proper subset of that key is also a key.