- From: Bijan Parsia <bparsia@cs.man.ac.uk>
- Date: Sun, 5 Sep 2010 15:21:16 +0100
- To: Bob MacGregor <bob.macgregor@gmail.com>
- Cc: Juan Sequeda <juanfederico@gmail.com>, Jitao Yang <jitao.yang@gmail.com>, semantic-web@w3.org, public-sparql-dev@w3.org
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. 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 14:21:54 UTC