Expressive Languages for Querying the Semantic Web

The Resource Description Framework (RDF) is the W3C recommendation data model to represent information about World Wide Web resources. An atomic piece of data in RDF is a Uniform Resource Identifier (URI). In the RDF data model, URIs are organized as RDF graphs, that is, labeled directed graphs where node labels and edge labels are URIs. The problem of querying RDF data is a central issue for the devel-opment of the Semantic Web. The query language SPARQL has become the standard language for querying RDF, since its standardization in 2008 [8]. However, the 2008 version of this language missed some important functionalities, and in particular, rea-soning capabilities to deal with RDF Schema (RDFS) [2] and Ontology Web Language (OWL) [6] vocabularies, that is, vocabularies with predefined semantics which can be used to derive logical conclusions from RDF graphs. To overcome these limitations, a new version of SPARQL, called SPARQL 1.1 [5], was recently released, which includes entailment regimes for RDFS and OWL vocabularies. Unfortunately, even if we focus on a lightweight profile of OWL 2 such as OWL 2 QL, the queries may become extremely complicated. This is mainly due to the fact that we are forced to encode the semantics of the RDFS and OWL vocabularies in the query. The aim of this work is to propose a language which separates the reasoning part from the actual query, and thus overcoming the above negative aspect of the existing query languages. Our language, called TriQ, is based on Datalog and some of its extensions; in particular, the members of the recently introduced Datalog family of knowledge representation and query languages [3]. In the rest of this short paper, we illustrate, via a simple example, some of the difficulties encountered when querying RDF data with SPARQL, which motivated us to design TriQ