A Source-to-Target Constraint Rewriting for Direct Mapping

Ratan Bahadur Thapa, Martin Giese

Abstract: Most of the existing structured digital information today is still stored in relational databases. That’s why it is important for the Semantic Web effort to expose the information in relational databases as RDF, or allow to query it using SPARQL. Direct mapping is a fully automated approach for converting well-structured relational data to RDF that does not require formulating explicit mapping rules [2, 8]. Along with the mapped RDF data, it is desirable to have a description of that data. Previous work [3, 8] has attempted to describe the RDF graph in terms of OWL axioms, which is problematic, partly due to the open world semantics of OWL. We start from the direct mapping suggested by Sequeda et al. [8], which integrates and extends the functionalities of proposal [10] and the W3C recommendation [2], and present a source-to-target semantics preserving rewriting of constraints in an SQL database schema to equivalent SHACL [7] constraints on the RDF graph. We thus provide a SHACL description of the RDF data generated by the direct mapping without the need to perform a costly validation of those constraints on the generated data. Following the approach of [8], we define the rewriting from SQL constraints to SHACL by a set of Datalog rules. We prove that our source-to-target rewriting of constraints is constraint preserving and weakly semantics preserving.