Introducing Nominals to the Combined Query Answering Approaches for EL

TL;DR

This paper extends combined query answering methods for the EL family of description logics to include nominals, enabling practical and sound query answering in a more expressive logic used in OWL 2 EL.

Contribution

It introduces a novel combined approach for ELHO that handles nominals and equality reasoning, filling a significant gap in existing methods.

Findings

Empirical evaluation shows practical applicability.

Method effectively handles nominals and equality reasoning.

Enables conjunctive query answering in a large OWL 2 EL fragment.

Abstract

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.