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[OK?] paper on RDF querying relevant to DAWG (and to current discussions)

From: Sergio Tessaris <tessaris@inf.unibz.it>
Date: Wed, 29 Mar 2006 09:18:46 +0200
Message-Id: <A021E7C2-C1AC-452E-8937-428A5F37ACEF@inf.unibz.it>
Cc: public-rdf-dawg-comments@w3.org, RDF Data Access Working Group <public-rdf-dawg@w3.org>
To: "Peter F. Patel-Schneider" <pfps@research.bell-labs.com>

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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

-- 
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Sergio Tessaris, Assistant Professor        Faculty of Computer Science
e-mail: tessaris@inf.unibz.it          Free University of Bozen-Bolzano
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tel: +39 0471 016 125 (fax 009)                    39100 Bolzano, Italy
Received on Wednesday, 29 March 2006 07:19:14 GMT

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