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

Date: 25 Feb 2006 08:06:32 -0800

Message-ID: <1140883592.060038.188390_at_z34g2000cwc.googlegroups.com>

Jan Hidders wrote:

*> 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.
*

Interesting.

Does this mean a canonical form for a key is impossible or undesirable?

My suspicion is that one won't be able to derive Armstrong's axioms from more general axioms of constraint propogation.

Also, I seem to recall from somewhere the result that while a normal form exists for FDs, that normal form is not guaranteed to be unique, or something like that. Anyone?

Marshall Received on Sat Feb 25 2006 - 17:06:32 CET