Re: properties as nodes etc.

On June 30, Thomas B. Passin writes:
> In this case the problem is equivalent, I think, to not knowing the
> primary key for a relational table.  Suppose we knew the key, that
> is, the set of properties that make each instance unique.  Then we
> could represent them in RDF as a bag.  Of course we need a predicate
> that can apply the key to the specific relationship.  Depending on
> what the key really was, we could tell if these two instances were
> actually one and the same.  This would be the constraint Enrico was
> talking about.

I don't really understand what are you saying. We do know the primary
key; the problem is that it is not made out of a single property, but
it is the whole set of properties.

> This suggests to me that the problem with this particular example is
> a data modeling problem and not a fundamental problem of
> representation.  Of course, there is no way in RDF to actually
> define such a primary key constraint beyond asserting the bag (the
> semantics of a primary key, in othe words), but the same can be said
> for almost all predicates that can be used in RDF statements..
> 
> The real problem about n-ary relationships in RDF is that, so far as
> I can see, you cannot distinguish between higher-order
> relationships, where one argument brings the others into a
> relationship (c.f. Sowa discussing Pierce's "thirdness"), and an
> ordinary first-order relationship containing a simple collection of
> properties that just happen to occur together.

Again, what are you saying is rather obscure. The additional
constraint I want to enforce is by no mean higher order, neither it
involves higher order relationships. It is pure first order:

\forall x1...xn . R(x1,...,xn) <-->
     \exists-unique z . RC(z) \and r1(z,x1) \and ... \and rn(z,xn)

> But maybe we do not have to be too discontented with RDF for not
> being able to represent higher order relationships when it is far
> from a full first-order system anyway.

????

cheers
-- e.

Enrico Franconi                     - franconi@cs.man.ac.uk
University of Manchester            - http://www.cs.man.ac.uk/~franconi/
Department of Computer Science      - Phone: +44 (161) 275 6170
Manchester M13 9PL, UK              - Fax:   +44 (161) 275 6204

Received on Monday, 1 July 2002 00:45:21 UTC