I4-D14 Exploring and Taming Existence in Rule-based RDFQueries

RDF is an emerging knowledge representation formalism proposed by the W3C. A central feature of RDF are blank nodes, which allow to assert the existence of an entity without naming for it. Despite the importance of blank nodes for RDF, many existing RDF query language have only insu cient support for blank nodes. We propose a query language for RDF, called RDFLog, with extensive blank node support. ¿e evaluation of RDFLog may be reduced to the evaluation of Datalog. ¿is allows to apply standard database technology to querying RDF. Our Experimental evaluation shows that our implementation scales well, even for large data sets. ¿e core feature of the reduction is Skolemisation and an new form of unSkolemisation. In contrast to previous de nitions of un-Skolemisation our un-Skolemisation has desirable symmetric properties to those of the Skolemisation. We de ne a hierarchy of syntactical restrictions of RDFLog with lower expressivity but better complexity, thereby showing the computational costs of blank nodes. We provide both an operational and a denotational semantics of RDFLog and show its soundness and completeness. Finally we discuss a notion of redundancy free answers, called lean answers, and state their complexity and an operational semantics. Keyword List RDFLog, blank nodes, existence, RDF, querying, rules Project co-funded by the European Commission and the Swiss Federal O ce for Education and Science within the Sixth Framework Programme.