Re: Notions of Type

J M Davitt wrote:
> David Cressey wrote:
> >
> > I'm surprised the PROJECT is such a problem. Maybe I should stay out of the
> > discussion, because this is a little over my head. But here goes, anyway:
> >
> > Why can't you define a "set of attributes" as a relation? I'm thinking
> > that an empty relation (one with no tuples) has exactly the same
> > information content as a "set of attributes". If you do that, why can't
> > you say,
> >
> > PROJECT <relation>, <empty relation> -> <relation>
> >
> >
> > Or have I violated some other aspect of the formalism?
>
> Wait a minute...
>
> PROJECT <relation> <set of attributes> -> <relation>
>
> right? And the question is, "Does this definition of
> PROJECT demonstrate lack of algebraic closure?" I think
>
> PROJECT <relation> <relation> -> <relation>
>
> is a step in the wrong direction and don't think such a
> thing is necessary for closure. I think that the fact
> that one of the operands and the result are relations is
> what provides closure.

Well, if one wants to be strict, one would have to disagree. And I'm the sort of guy who likes to be strict.

Marshall Received on Fri Aug 18 2006 - 08:01:02 CEST