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

From: Brian Selzer <brian_at_selzer-software.com>
Date: Thu, 02 Nov 2006 14:34:18 GMT
Message-ID: <Kln2h.101$Mw.94@newssvr11.news.prodigy.com>

"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. 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 - 08:34:18 CST