Re: What are the differences between the terms, CANDIDATE KEY, PRIMARY KEY, SUPER KEY, COMPOSITE KEY?
Date: Sat, 25 Feb 2006 10:54:18 GMT
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.
- Jan Hidders