W3C home > Mailing lists > Public > www-rdf-interest@w3.org > August 2004

Re: bc inferencing on optional query patterns

From: Adrian Walker <adrianw@snet.net>
Date: Thu, 19 Aug 2004 11:52:53 -0400
Message-Id: <5.0.2.1.2.20040819115038.0244eb60@pop.snet.net>
To: phil@phildawes.net
Cc: www-rdf-interest@w3.org

Phil --

The example RDFQueryLangComparison1 at www.reengineeringllc.com may be 
useful....

If you run the example in a browser, and ask for explanations of results, 
you can see the reasoning chains in somewhat human-friendly notation.

HTH,  -- Adrian


At 07:45 AM 8/19/04 +0100, you wrote:

>Hi RDF-Interest, Hi Steve,
>
>I'm developing an experimental store for smushing ifp resources, which
>backs onto a relational database (similar to 3store).
>
>I've been trying to implement backward-chaining subproperty
>inferencing with BRQL-style optional query patterns, and am coming a
>bit unstuck.
>
>E.g. take the following query, which gets all http://example.com/foo's
>props and objects, and if the object is a resource also gets an
>rdfs:label for it:
>
>select ?obj, objlabel
>where    (<http://example.com/foo> ?prop ?obj)
>OPTIONAL (?obj rdfs:label ?objlabel)
>
>I want to do subproperty inferencing on the optional pattern to return
>e.g. foaf:name labels (foaf:name is a subproperty of rdfs:label).
>As in the 3store paper[1], I am using the idiom:
>
>?subj ?prop ?obj
>->
>?subj ?tmp ?obj
>?tmp SUBPROPERTYOF ?prop
>
>
>But that yields:
>
>select ?obj, ?objlabel
>where    (<http://example.com/foo> ?prop ?obj)
>OPTIONAL (?obj ?tmp ?objlabel)
>OPTIONAL (?tmp rdfs:subPropertyOf rdfs:label)
>
>Which introduces a variable that exists only in the optional
>clauses. I believe this makes the query un-deterministic - correct?
>(it certainly doesnt work when it's translated into a relational sql
>query with left-joins).
>N.B. this sort of query also isnt allowed by e.g. sesame.
>
>So the only other solution is to do:
>select ?obj, ?objlabel
>where    (<http://example.com/foo> ?prop ?obj)
>          (?tmp rdfs:subPropertyOf rdfs:label)
>OPTIONAL (?obj ?tmp ?objlabel)
>
>When this query evaluates, you get n*results where n is the number of
>subproperties of rdfs:label. Most of these have 'NULL' values for the
>objlabel (which correspond to subproperties of rdfs:label that don't
>match a value).
>
>i.e. I want:
>
>?obj (?tmp)         ?objlabel
>---------------------------
>1234  foaf:name   "fred blogs"
>2345  rdfs:label  "2345 label"
>
>
>but I get:
>
>?obj (?tmp)       ?objlabel
>---------------------------
>1234  rdfs:label  NULL
>1234  foaf:name   "fred blogs"
>1234  anotherprop NULL
>2345  rdfs:label  "2345 label"
>2345  foaf:name   NULL
>2345  anotherprop NULL
>
>
>This doesnt scale very well, as in a big store there will be *lots* of
>subproperties of rdfs:label. Am I missing something obvious? How do
>other stores tackle this problem?
>
>Many thanks,
>
>Phil
>
>[1]
>http://km.aifb.uni-karlsruhe.de/ws/psss03/proceedings/harris-et-al.pdf
Received on Thursday, 19 August 2004 15:52:25 UTC

This archive was generated by hypermail 2.3.1 : Wednesday, 5 February 2014 07:14:57 UTC