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[SKOS, SPARQL, ALL] Closure and SPARQL

From: McBride, Brian <brian.mcbride@hp.com>
Date: Sat, 3 Dec 2005 21:16:45 -0000
Message-ID: <DE62D3D0BDEF184FBB5089C7D387C374556552@sdcexc04.emea.cpqcorp.net>
To: <public-swbp-wg@w3.org>

I have been reviewing SPARQL last call WD on the plane as promised, and
wanted to return to an issue that David Wood had previously raised.  (No
URLs available at 30k feet - sorry)

SPARQL does not support the expression of property closures, e.g.
transitive closure.

To ground this in BP work, consider an RDF graph that includes
dc:subject properties whose values come from a SKOS taxonomy.  You
cannot express directly in SPARQL a query to find all the resources
whose dc:subject is a (possibly indirect) narrower term of some term T.

I earlier suggested that SPARQL does not have to support transitive
closure because the graph can do it.  There can be an inferencing graph
which computes the transitive closure of skos:narrower.  If you query
that graph you query the transitive closure relationship.  If you query
the ground graph you get the direct, well at least directly asserted,
relationship.  The question arises, whether there is a need to
distinguish between the direct relationship and the closure
relationship.  Are these different properties.  Should we define
skos:inNarrowerClosure (or something more appealingly named).

In terms of the plumbing, this is not necessary.  SPARQL supports
querying over multiple graphs and so can support querying the ground
graph to get at the direct relationship and an inferred graph to get at
the closed one.

However it strikes me that the direct relationship and the closure are
different relationships and it might be best practice, perhaps
generally, perhaps under certain circumstances, to define both.  I
believe the distinction to be useful.  If I am creating a graphical
representation of a SKOs taxaonomy, I want to query for the direct
relationship.  But if I'm searching for relevant resources, I probably
want the closure relationship.  If it can be useful to make this
distinction, there may be value in defining property to relate the
direct property to its closure property, e.g. ex;isTransitiveClosureOf.

I'm feeling a bit out of my depth here.  I expect this all got worked
out a while ago, but just thought I'd check.

Brian
Received on Saturday, 3 December 2005 21:16:50 UTC

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