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

saturnlee_at_yahoo.com wrote:
> I have a basic question.
> Suppose there are 3 attributes: A,B and C.
> And there are no FD(trivial and non trivial )
>
> What 's the key for it? ABC or nothing???
>

[I recently learned about part of this on this news group.]

You've got two of the possibilities covered.

It's axiomatic, but the set of all attributes -- {A, B, C}, in this case -- form a key and prevent duplicates.

If subsets of the attributes are keys, then something other than "all attributes" is a candidate key and the "all attributes" key is a super key.

It may be that {A} is a candidate key while {B} (or {B, C} or something else) is/are, simultaneously, candidate key(s).

If a candidate key is declared as {}, there may be no other candidate key and any relation value may consist of no more than one tuple -- but all non-empty sets of attributes form superkeys.

The fuzziness? "Candidate keys" are those which are unique and irreducible; this is what some folks mean when they say "key." Super keys are always called "super keys," as far as I know.

I used to have brain-disconnect when someone said "key" when I thought they should have said "candidate key," but I have come to realize that "key" should be understood as candidate key. Received on Mon Oct 30 2006 - 00:16:05 CET