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

From: vldm10 <vldm10_at_yahoo.com>
Date: 2 Nov 2006 14:00:41 -0800
Message-ID: <1162504841.417506.219660_at_m7g2000cwm.googlegroups.com>


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.

Very good.

>A relation is a set of tuples.

> Because it is a set, there can be no duplicates.
>
> A relation does not contain entities:

Very good!

>it contains statements about entities.

I am not sure that you are not confused here?

> 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 - 23:00:41 CET

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