Abstract
We present an extension of the ontology-based data access platform Ontop that supports answering SPARQL queries under the OWL 2QL direct semantics entailment regime for data instances stored in relational databases. On the theoretical side, we show how any input SPARQL query, OWL 2QL ontology and R2RML mappings can be rewritten to an equivalent SQL query solely over the data. On the practical side, we present initial experimental results demonstrating that by applying the Ontop technologies—the tree-witness query rewriting, T -mappings compiling R2RML mappings with ontology hierarchies, and T -mapping optimisations using SQL expressivity and database integrity constraints—the system produces scalable SQL queries.