- From: Adrian Walker <adriandwalker@gmail.com>
- Date: Thu, 21 Aug 2008 13:58:33 -0400
- To: "Kingsley Idehen" <kidehen@openlinksw.com>
- Cc: axel.polleres@deri.org, public-sparql-dev@w3.org, public-lod@w3.org
- Message-ID: <1e89d6a40808211058s2d816e7eqfab577879f7203f2@mail.gmail.com>
Hi Kingsley --
You wrote...
Re. SPARQL & Aggregates, see:
http://esw.w3.org/topic/SPARQL/Extensions/Aggregates
Yes, that shows that some folks are thinking about the issues.
But the fundamental problem is (as stated at the foot of that page) that
different implementations of SPARQL aggregates are going ahead without any
spec saying *what *should be computed. This is the sad SQL history
repeating itself, except it's going to be worse with on-the-fly linked RDF
data than it was for SQL-with-known-data-tables.
Cheers, -- Adrian
Internet Business Logic
A Wiki and SOA Endpoint for Executable Open Vocabulary English over SQL and
RDF
Online at www.reengineeringllc.com Shared use is free
Adrian Walker
Reengineering
On Thu, Aug 21, 2008 at 1:47 PM, Kingsley Idehen <kidehen@openlinksw.com>wrote:
> 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><
>> 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<http://www.openlinksw.com/blog/%7Ekidehen>
> President & CEO OpenLink Software Web: http://www.openlinksw.com
>
>
>
>
>
Received on Thursday, 21 August 2008 17:59:09 UTC