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)
> >
>
> The above example is not a relation. A relation is a set of tuples.
> Because it is a set, there can be no duplicates.
>
> A relation does not contain entities: it contains statements about entities.
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.
Received on Thu Nov 02 2006 - 19:15:51 CST
