- From: Sergio Tessaris <tessaris@inf.unibz.it>
- Date: Wed, 29 Mar 2006 09:18:46 +0200
- To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>
- Cc: public-rdf-dawg-comments@w3.org, RDF Data Access Working Group <public-rdf-dawg@w3.org>
--- Please respond indicating whether you are or are not satisfied with this response. If you are, you can help our issue tracking system by prefixing the subject of your response with [CLOSED] (where this subject has [OK?]). --- The mentioned papers indeed present a coherent and well defined framework for the problem of querying RDF graphs. However, the query language proposed is somewhat restricted and doesn't fulfil the requirements for SPARQL (see [UCNR]). Moreover, in its basic definitions is not far away from the current version of the SPARQL document. In fact, a tableau as defined in the paper can be seen as a CONSTRUCT query containing a single graph pattern, where the head and the body of the tableau are the template and the graph pattern respectively. The semantic definitions in Section 4.2 of the paper are substantially equivalent to those of the latest SPARQL document. In particular, the definition of pre-answer correspond to the SPARQL answer set (provided that valuations are considered instead of graphs). Note that the notion of answers to a query is defined in terms of subgraph homomorphism; so, although it works well with RDF(S) has problems in scaling up with more expressive ontology languages (see owlDisjunction issue at http://www.w3.org/2001/sw/DataAccess/ issues#owlDisjunction). In the paper, two different semantics are considered for the result of a query as the combination of the graphs in the so called pre- answer, namely the union or the (RDF) merge. In SPARQL, the CONSTRUCT operator adopts the merge semantics. Constraints, as introduced in the paper correspond to the FILTER operator in SPARQL. The paper introduces the idea of premises for a query, which can be easily handled by SPARQL by means of the ability of querying multiple graphs. Regards, --sergio [UCNR] RDF Data Access Use Cases and Requirements , K. Clark, Editor, W3C Working Draft. Latest version available at http://www.w3.org/TR/ rdf-dawg-uc/ . On Sunday 18 September 2005 17:02, Peter F. Patel-Schneider wrote: > Hi: > > A couple of days ago I came across the following paper: > > Foundations of Semantic Web Databases. > Claudio Gutierrez, Carlos Hurtado - Universidad de Chile; > Alberto Mendelzon - University of Toronto. > PODS 2004, pp. 95 - 106. > > The paper refers to an earlier paper, "Formal aspects of querying RDF > databases", "for the case with no rdfs vocabulary" that was > presented at > the First International Workshop on Semantic Web and Databases co- > located > with VLDB held at Berlin, Germany, September 7-8, 2003. > > These papers have a very good set of definitions for querying RDF. > Although the second, more developed, paper concentrates on the case > of RDFS > its solutions appear to be directly relevant to the Data Access Query > Working Group. > > > Peter F. Patel-Schneider > Bell Labs Research -- ----------------------------------------------------------------------- Sergio Tessaris, Assistant Professor Faculty of Computer Science e-mail: tessaris@inf.unibz.it Free University of Bozen-Bolzano http://www.inf.unibz.it/~tessaris Piazza Domenicani 3 tel: +39 0471 016 125 (fax 009) 39100 Bolzano, Italy
Received on Wednesday, 29 March 2006 07:19:14 UTC