Re: Functional Dependencies > Uniqueness Constraints

Bob Badour wrote:
> paul c wrote:
>
> > Marshall wrote:
> >
> >> ... (And of course there must be a rule that
> >> says every base table must have at least one functional
> >> dependency in which the union of the determinant set
> >> and the dependent set equals the set of attributes. (This
> >> restriction is sufficient to ensure every base table is a
> >> relation; is it necessary?))
> >> ...
> >
> > I would say not necessary. If a table is a representation of a
> > relation, then I`d think that even if no rule is stated, by definition
> > the union of the attributes is a CK, eg., if there is no stated
> > determinant set, all the attributes are in the dependent set. I can`t
> > think why one would want to state this, shouldn`t a dbms assume itÉ
>
> I think you have determinant and dependent reversed. The attributes of a
> candidate key are the determinant set, and the remaining attributes are
> each dependent attributes. Thus, if no other key is specified, all
> attributes are in the determinant set and the set of dependent
> attributes is empty.
>
> What Marshall stated is an invariant of every relation for every
> candidate key. In fact, it seems to me Marshall's statement is just a
> restatement of candidate keys, but there could be subtleties I miss.

Yes, that's exactly what it is: a restatement of the requirement for at least one candidate key, but stated in the terminology of functional dependencies. The requirement comes from the definition of set, of course, so any constraint of whatever form that satisfies the definition of set (which is to say, no duplicates) is sufficient.

I think the way I stated it is overly strong for what is necessary to meet the definitional requirement. However using any weaker requirement *for base tables* seems like a really, really bad idea. On the other hand, for a result set, the FDs are whatever they are.

Marshall Received on Thu Aug 31 2006 - 01:35:31 CEST