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Re: properties as nodes etc.

From: Thomas B. Passin <tpassin@comcast.net>
Date: Sun, 30 Jun 2002 22:36:43 -0400
To: www-rdf-logic@w3.org
Message-id: <000c01c220a8$2359fdd0$fe193044@tbp1>

[Enrico Franconi]

>The problem is: how to write this constraint in RDF?
>
> If you ask me an example on the fly, I tell you the following:
>
> teach(john,cs336,sem2,2001/2002)
> teach(john,_z,sem2,2001/2002)
>
> which becomes, reified:
>
> teach(_x)
> prof(_x,john)
> course(_,cs336)
> semester(_,sem2)
> acad-year(_,2001/2002)
>
> teach(_y)
> prof(_y,john)
> course(_y,_z)
> semester(_y,sem2)
> acad-year(_y,2001/2002)
>
> How can you tell whether _x and _y denote distinct URIs?

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.

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.

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,

Tom P
Received on Sunday, 30 June 2002 22:35:57 GMT

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