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Mapping relational databases to RDF is a fundamental problem for thedevelopment of the Semantic Web. We present a solution, inspired by draftmethods defined by the W3C where relational databases are directly mapped toRDF and OWL. Given a relational database schema and its integrity constraints,this direct mapping produces an OWL ontology, which, provides the basis forgenerating RDF instances. The semantics of this mapping is defined usingDatalog. Two fundamental properties are information preservation and querypreservation. We prove that our mapping satisfies both conditions, even forrelational databases that contain null values. We also consider two desirableproperties: monotonicity and semantics preservation. We prove that our mappingis monotone and also prove that no monotone mapping, including ours, issemantic preserving. We realize that monotonicity is an obstacle for semanticpreservation and thus present a non-monotone direct mapping that is semanticspreserving.

On Directly Mapping Relational Databases to RDF and OWL (Extended Version)