  Mapping relational databases to RDF is a fundamental problem for the
development of the Semantic Web. We present a solution, inspired by draft
methods defined by the W3C where relational databases are directly mapped to
RDF and OWL. Given a relational database schema and its integrity constraints,
this direct mapping produces an OWL ontology, which, provides the basis for
generating RDF instances. The semantics of this mapping is defined using
Datalog. Two fundamental properties are information preservation and query
preservation. We prove that our mapping satisfies both conditions, even for
relational databases that contain null values. We also consider two desirable
properties: monotonicity and semantics preservation. We prove that our mapping
is monotone and also prove that no monotone mapping, including ours, is
semantic preserving. We realize that monotonicity is an obstacle for semantic
preservation and thus present a non-monotone direct mapping that is semantics
preserving.
