Re: Functional Dependency to constrain a relation to exactly one element?

From: vc <boston103_at_hotmail.com>
Date: 1 Oct 2006 19:13:49 -0700
Message-ID: <1159755229.423891.37190@e3g2000cwe.googlegroups.com>

Marshall wrote:
> vc wrote:
> > Marshall wrote:
> > >
> > > Well, we could always pair an empty determinant set FD with
> > > an empty existential constraint. Or, given a candidate key k
> > > of relation R, we could perhaps say
> > >
> > > exists R.k: forall R.k': k = k'
> > >
> > > (using x' to indicate a different binding of attribute x)
> > >
> > > Of course, you could make a good case that the above
> > > isn't "simple".
> >
> > One would make a good case that it is not "existential" (why not
> > "universal" ?) ;)

>

> Well, that's a fair question. What do you call a constraint of
> the form "exists x: forall y: P(x,y)". What about the other order:
> "exists y: forall x: P(x,y)"? I'm inclined to call them by the
> type of the outermost quantifier, but I've never seen this
> addressed in a book.
> Maybe the terminology isn't that important.

Probably it is not.

>

> Hey, maybe we could string the quantifiers together, so
> we could have a uni-uni-exi-quantified statement.
> Or exi-uni-quantified.

You can use arithmetical hierarchy terminology based on quantifier block alternations where the formula would be classified as "sigma_0_2" since it starts with E (sigma) and has one alternation. However, this kind of jargon is hardly helplful here.

>
>
> Marshall
Received on Sun Oct 01 2006 - 21:13:49 CDT