Introducing Nominals to the Combined Query Answering Approaches for EL
This work addresses a gap in query answering for a large fragment of OWL 2 EL, enabling practical applications in semantic web domains.
The paper tackles the problem of conjunctive query answering over ontologies with nominals, which previous combined approaches could not handle, and presents a method for ELHO that is shown to be suitable for practical application.
So-called combined approaches answer a conjunctive query over a description logic ontology in three steps: first, they materialise 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 answers. 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 answering approach for ELHO---a logic that contains all features of the OWL 2 EL standard apart from transitive roles and complex role inclusions. This extension is nontrivial because 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 conjunctive query answering in a large fragment of OWL 2 EL.