Introducing Nominals to the Combined Query Answering Approaches for EL

So-called combined approaches answer a conjunctive query over a description logic ontology in three steps: first, they ma-terialise certain consequences of the ontology and the data; second, they evaluate the query over the data; and third, they filter the result of the second phase to eliminate unsound an-swers. Such approaches were developed for various members of the DL-Lite and the EL families of languages, but none of them can handle ontologies containing nominals. In our work, we bridge this gap and present a combined query an-swering approach for ELHOr⊥—a logic that contains all fea-tures of the OWL 2 EL standard apart from transitive roles and complex role inclusions. This extension is nontrivial be-cause nominals require equality reasoning, which introduces complexity into the first and the third step. Our empirical evaluation suggests that our technique is suitable for practical application, and so it provides a practical basis for conjunc-tive query answering in a large fragment of OWL 2 EL