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Re: soliciting your favorite (public) SPARQL queries!

From: Kingsley Idehen <kidehen@openlinksw.com>
Date: Thu, 21 Aug 2008 13:47:41 -0400
Message-ID: <48ADAA3D.5060509@openlinksw.com>
To: Adrian Walker <adriandwalker@gmail.com>
CC: Axel Polleres <axel.polleres@deri.org>, public-sparql-dev@w3.org, public-lod@w3.org

Adrian Walker wrote:
> Hi Axel --
>
> Good to see some thinking about fundamentals.  
>
> The omission of negation from the SPARQL spec (and hence the need for 
> your ingenious workarounds) seems to be based on a confusion that can 
> perhaps be explained away like this....
>
> What some semantic web folks seem to want is that when new facts are 
> added, old conclusions don't go away.  They want things to be 
> monotonic, and they therefore deprecate SQL-style negation-as-failure 
> (NAF).
>
> Now suppose an old conclusion p depends on ~r  in a consistent theory, 
> and that an update r is pending.
>
> We could just add r.  p would still hold, but the new theory has both 
> r and ~r.  It's inconsistent.  That means  that a naive theorem prover 
> can prove  absolutely anything from it.  A better theorem prover would 
> probably refuse to compute with it.  Neither is a desirable outcome.
>
> But wait.  In most practical circumstances, adding r is a way of 
> saying that ~r should be removed.  So, take the update to mean "add r 
> and also delete ~r".   The new theory is consistent, and p no longer 
> holds.
>
> So, the price of keeping consistency through an update is that an old 
> conclusion p may no longer  be entailed.  Under consistent update, 
> using classical logic and using NAF lead to the */same /*behavior.
>
> If we use Clark's result [1] to view a logic program with NAF as 
> simply shorthand for a set of clauses in classical logic, the above 
> starts to look kind of obvious.
>
> A similar argument could be advanced for the inclusion of aggregation 
> in an extended SPARQL spec.  Now is perhaps a good time to avoid an 
> error that the SQL folks made -- the results from SQL aggregations are 
> implementation dependent.  That's a bad idea for SQL, and a terrible 
> one for on-the-fly linked data and the Semantic Web.
>
> Hope this helps.
>
>                                          -- Adrian
>
>
> [1]  http://www.doc.ic.ac.uk/~klc/NegAsFailure.pdf 
> <http://www.doc.ic.ac.uk/%7Eklc/NegAsFailure.pdf>
>
> Internet Business Logic
> A Wiki and SOA Endpoint for Executable Open Vocabulary English over 
> SQL and RDF
> Online at www.reengineeringllc.com 
> <http://www.reengineeringllc.com>    Shared use is free
>
> Adrian Walker
> Reengineering
>
>
> On Thu, Aug 21, 2008 at 11:51 AM, Axel Polleres 
> <axel.polleres@deri.org <mailto:axel.polleres@deri.org>> wrote:
>
>
>     Tackling the question from the more theoretical side,
>     I like non-monotonic SPARQL queries like the ones modeling set
>     difference.
>
>     E.g.
>     "Give me all persons *without* an email address" in a certain FOAF
>     graph.
>
>
>     i) It is already folklore, that you can do that with using the
>     !bound() filter outside an optional, i.e.
>
>     SELECT ?X
>     FROM G
>     WHERE { ?X a foaf:Person
>            OPTIONAL { ?X foaf:mbox ?M}
>            FILTER (! bound(?X) ) }
>
>
>     ii) What some people might find surprising is that I can achieve
>     the same result without using a FILTER, more generally that I can
>     express
>
>     SELECT ?X
>     FROM G
>     FROM NAMED <boundchecker.rdf>
>     WHERE
>      {
>        { ?X a foaf:Person OPTIONAL{ ?X foaf:mbox ?M} }
>        GRAPH <boundchecker.rdf>{ ?M :is :unbound }
>      }
>
>     where <boundchecker.rdf> is the graph containing the single triple
>
>       _:b :is :unbound.
>
>     Maybe requires some thinking, but is a nice example :-)
>
>     (Short explanation: the blanknode in Graph <boundchecker.rdf> only
>     matches to unbound variables from the optional patttern. Note that
>     non-well-designed OPTIONAL patterns are not commutative, see [1].
>     Actually, [1] "kind of" conjectured that non-well-designed
>     patterns are useless, but - as this query shows - they aren't
>     really entirely useless.)
>
>     Axel
>
>     1. http://iswc2006.semanticweb.org/items/Arenas2006bv.pdf
>
>
>     p.s.: Since I didn't see a similar one before, I claim copyright
>     for that one, basically, it is very easily generalizable to model
>     arbitrary queries  SELECT ... P WITHOUT P'
>      ;-)
>
>
>
>     Lee Feigenbaum wrote:
>
>         Hi everyone,
>
>         I'm putting together a "SPARQL by Example" tutorial, which is,
>         as the name suggests, a step-by-step introduction to SPARQL
>         taught almost entirely through complete, runnable SPARQL queries.
>
>         So far, I've gathered a great deal of example queries myself,
>         but I know that many subscribers to these lists probably have
>         favorite queries of their own that you might be willing to
>         share with me.
>
>         I'm looking for:
>
>         1) SPARQL queries
>         2) ...that can be run by anyone (no private data sets)
>         3a)...either by running the query against a public SPARQL endpoint
>         3b)...or by using a public SPARQL endpoint that will fetch
>         HTTP-accessible RDF data (e.g. sparql.org <http://sparql.org>
>         or demo.openlinksw.com <http://demo.openlinksw.com>)
>         4) ...that answers a real* question
>         5) ...and that is fun!**
>
>         * real is in the eye of the beholder, I imagine, but I'm not
>         looking for  "finds the predicates that relate ex:s and ex:o
>         in this sample RDF graph"
>
>         ** fun is also in the eye of the beholder. fun can be a query
>         on fun data; a clever query that may illustrate a particular
>         SPARQL construct ("trick"); a query that integrates
>         interesting information; a query with surprising results; etc.
>
>         thanks to anyone who is able to contribute!
>         Lee
>
>         PS I plan to make the tutorial slides available online under
>         an appropriate CC license once they are completed.
>
>
>
>     -- 
>     Dr. Axel Polleres, Digital Enterprise Research Institute (DERI)
>     email: axel.polleres@deri.org <mailto:axel.polleres@deri.org>
>      url: http://www.polleres.net/
>
>     Everything is possible:
>     rdfs:subClassOf rdfs:subPropertyOf rdfs:Resource.
>     rdfs:subClassOf rdfs:subPropertyOf rdfs:subPropertyOf.
>     rdf:type rdfs:subPropertyOf rdfs:subClassOf.
>     rdfs:subClassOf rdf:type owl:SymmetricProperty.
>
>
Adrian,

Re. SPARQL & Aggregates, see: 
http://esw.w3.org/topic/SPARQL/Extensions/Aggregates


-- 


Regards,

Kingsley Idehen	      Weblog: http://www.openlinksw.com/blog/~kidehen
President & CEO 
OpenLink Software     Web: http://www.openlinksw.com
Received on Thursday, 21 August 2008 17:48:21 GMT

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