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Re: First order logic and SPARQL

From: Axel Polleres <axel.polleres@deri.org>
Date: Sun, 5 Sep 2010 16:17:13 +0100
Cc: "Bob MacGregor" <bob.macgregor@gmail.com>, "Juan Sequeda" <juanfederico@gmail.com>, "Jitao Yang" <jitao.yang@gmail.com>, <semantic-web@w3.org>, <public-sparql-dev@w3.org>
Message-Id: <06A68B64-DB4C-4B43-856E-CB130B76B7AC@deri.org>
To: Bijan Parsia <bparsia@cs.man.ac.uk>
> > The problem with SPARQL stems from the OPTIONAL operator.  A mantra 
> > of RDF has been that it
> > has open world semantics.  The OPTIONAL operator is inherently non-
> > monotonic.
> 
> ?? I don't think so. I'd be interested in a reference.

Obviously OPTIONAL is nonmomotonic and in fact, NOT EXISTS can be emulated not only 
with the widely known OPTIONAL + FILTER !Bound() trick (see [1] Query #13 for an example),
but you actually don't need the FILTER even (see Query #14 in the same tutorial [1]).

In SPARQL 1.1 we will very likelty have explicit MINUS/NOT EXISTS operators such that 
you don't need those tricks anymore to model negation, see [2] Queries #16, #16b, 16#c.  

(Thanks Lee for his excellent tutorials, BTW!) 

Axel

1. http://personnel.univ-reunion.fr/fred/Enseignement/SW/SPARQL-by-example/
2. http://www.cambridgesemantics.com/2008/09/sparql-by-example/



On 5 Sep 2010, at 15:21, Bijan Parsia wrote:

> On 5 Sep 2010, at 02:29, Bob MacGregor wrote:
> [snip]
> >
> > Yes, really.  It sounds very much like you have defined/referenced a 
> > cleaned-up version of SPARQL which
> > unfortunately does not reflect the real-world semantics.
> 
> http://www.w3.org/TR/rdf-sparql-query/#sparqlDefinition
> 
> The semantics of (a good chunk) of the algebra is in terms of the 
> relational algebra.
> 
> The formalization is based on this paper:
>         http://arxiv.org/pdf/cs/0605124v1
> 
> I wouldn't conflated declarative (or formal) semantics with model 
> theoretic.
> 
> > The problem with SPARQL stems from the OPTIONAL operator.  A mantra 
> > of RDF has been that it
> > has open world semantics.  The OPTIONAL operator is inherently non-
> > monotonic.
> 
> ?? I don't think so. I'd be interested in a reference.

Obviously OPTIONAL is nonmomotonic and in fact, NOT EXISTS can be emulated not only 
with the widely known OPTIONAL + FILTER !Bound() trick, but also 

> 
> Note that non-communitivity doesn't imply non-monotinicity. After all, 
> implication is non-communitive. Optional is defined in terms of left-
> outer join.
> 
> > A few of us devised
> > a closed-world semantics for OPTIONAL, but the open-world advocates 
> > rejected the notion, favoring instead
> > a procedural semantics.
> 
> The meaning is the meaning, regardless of the presentation of that 
> semantics.
> 
> > Not only are arguments to OPTIONAL defined to be order-dependent 
> > (analogous to a series
> > of if-then-else clauses),
> 
> Like implications in first order logic.
> 
> > but the SPARQL AND operator became polluted as well -- changing the 
> > order of conjuncts
> > that contain OPTIONALs can change the semantics of a SPARQL query.  
> > I don't have examples available
> > on the tip of my tongue, but a talk I gave a year ago at SEMTECH had 
> > an example, and there are many
> > others out there who should be able to furnish examples.
> 
> Can we dig this out?
> 
> > It would be a great service to the RDF community if you or someone 
> > would propose a semantically
> > well-founded variant of SPARQL (call it SPARQLL for "logical 
> > SPARQL", or whatever).
> 
> I think that's called SPARQL/1.0.
> 
> > It would necessarily
> > have closed-world semantics (as does Datalog).
> 
> Well, unbound requires epistemic reflection, but I don't think 
> OPTIONAL does per se.
> 
> There's a lot of tricky parts of any query language because of e.g., 
> the need to report and control answers. It's perfectly reasonable to 
> quarrel with choices you don't like, but I think we should be a bit 
> more careful about the source of the problems. SPARQL/1.0 has a pretty 
> reasonable and standard formalization.
> 
> Cheers,
> Bijan.
> 
> 
Received on Sunday, 5 September 2010 15:17:50 GMT

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