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Home -> Community -> Usenet -> comp.databases.theory -> Re: What are the differences between the terms, CANDIDATE KEY, PRIMARY KEY, SUPER KEY, COMPOSITE KEY?

Re: What are the differences between the terms, CANDIDATE KEY, PRIMARY KEY, SUPER KEY, COMPOSITE KEY?

From: Jan Hidders <jan.hidders_at_REMOVETHIS.pandora.be>
Date: Sat, 25 Feb 2006 10:54:18 GMT
Message-ID: <uHWLf.278635$PC2.8409285@phobos.telenet-ops.be>


Marshall Spight wrote:
>
> But it strikes me that there is an air of triviality in any FD where
> some specific attribute appears both on the left side *and* on the
> right side. We can apply decomposation to AB->BC and get
>
> 1) AB->B
> 2) AB->C
>
> 1) is trivial. AB->BC doesn't say anything that isn't said by AB->C.
>
> So if we describe a key as being an FD X->Y, where Y is the
> header, then we necessarily (unless X is empty) have attributes
> that appear in both X and Y. It strikes me that this could be reduced
> by replacing Y with (header - X).

Correct. But restricting yourself to only non-intersecting FDs has also its problems. Armstrong's rules would, for example, no longer be complete.

Received on Sat Feb 25 2006 - 04:54:18 CST

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