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Re: properties as nodes etc.

From: Thomas B. Passin <tpassin@comcast.net>
Date: Mon, 01 Jul 2002 01:01:01 -0400
To: www-rdf-logic@w3.org
Message-id: <000401c220bc$4bf23460$fe193044@tbp1>

[Enrico Franconi]

>
> 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.
>
I see, I couldn't be sure from your post.  I don't think it changes the
argument, though, to have the primary key equal the set of all columns in a
table (or equvalently, all properties in the relationship), as long as which
properties are allowed is unchangable.  Seth was showing templates for n-ary
relationships, and for templates to work with RDF there has to be some
predicate that connects a template to its instance.  RDF cannot (of course)
express the semantics of that connection.

> > 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:
>

I realize that, I was going beyond your example following some other
thoughts that this thread kicked off.

> \forall x1...xn . R(x1,...,xn) <-->
>      \exists-unique z . RC(z) \and r1(z,x1) \and ... \and rn(z,xn)
>
Received on Monday, 1 July 2002 01:00:15 GMT

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