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SPARQL is the standard language for querying RDF data. In this article, we address systematically the formal study of the database aspects of SPARQL, concentrating in its graph pattern matching facility. We provide a compositional semantics for the core part of SPARQL, and study the complexity of the evaluation of several fragments of the language. Among(More)
Navigational features have been largely recognized as fundamental for graph database query languages. This fact has motivated several authors to propose RDF query languages with navigational capabilities. In particular, we have argued in a previous paper that nested regular expressions are appropriate to navigate RDF data, and we have proposed the nSPARQL(More)
The Semantic Web is the initiative of the W3C to make information on the Web readable not only by humans but also by machines. RDF is the data model for Semantic Web data, and SPARQL is the standard query language for this data model. In the last ten years, we have witnessed a constant growth in the amount of RDF data available on the Web, which have(More)
SPARQL -the standard query language for querying RDF- provides only limited navigational functionalities, although these features are of fundamental importance for graph data formats such as RDF. This has led the W3C to include the property path feature in the upcoming version of the standard, SPARQL 1.1.We tested several implementations of SPARQL 1.1(More)
Static analysis is a fundamental task in query optimization. In this paper we study static analysis and optimization techniques for SPARQL, which is the standard language for querying Semantic Web data. Of particular interest for us is the optionality feature in SPARQL. It is crucial in Semantic Web data management, where data sources are inherently(More)
The Resource Description Framework (RDF) is the standard data model for representing information about World Wide Web resources. Jointly with its release as Recommendation of the W3C, the natural problem of querying RDF data was raised. In the last years, the language SPARQL has become the standard query language for RDF and, in fact, a W3C Recommendation(More)
Nested regular expressions (NREs) have been proposed as a powerful formalism for querying RDFS graphs, but not too much investigation on NREs has been pursued in a more general graph database context. In this paper we study the relative expressiveness of NREs by comparing it with the language of conjunctive two-way regular path queries (C2RPQs), which is(More)
In the recent years, a lot of attention has been paid to the development of solid foundations for the composition and inversion of schema mappings. In this paper, we review the proposals for the semantics of these crucial operators. For each of these proposals, we concentrate on the three following problems: the definition of the semantics of the operator,(More)