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

Marshall wrote:
> vc wrote:
> > Jan Hidders wrote:
> > > Marshall wrote:
> > [...]
> > > > Is it possible to have a functional dependency > > [...]
> > > > If it can't be done with an FD, can we do it with a constraint?
> > >
> > > Of course. In first-order logic you can specify this (i.e. that there
> > > is at least one tuple in the relation) with a simple existential
> > > statement.
> >
> > What simple existential statement do you have in mind ?
>
> If we want to ensure that relation R has at least one
> element, *any* existential constraint on R will do it.

Right, but it was not your original question. You asked about a constraint "that will constrain R to *exactly* one row?". Clearly, "*any* existential constraint on R" won't do that.

> The existential constraint on R that binds no variables to
> the empty expression would do it.
>
>
> Marshall
Received on Sun Oct 01 2006 - 16:40:22 CEST