Conjunctive Queries for EL with Role Composition

EL is a rather expressive description logic (DL) that still admits polynomial time inferencing for many reasoning tasks. Conjunctive queries are an important means for expressive querying of DL knowledge bases. In this paper, we address the problem of computing conjunctive query entailment for EL knowledge bases. As it turns out, querying unrestricted EL is actually undecidable, but we identify restrictions under which query answering becomes decidable and even tractable. To the best of our knowledge, the presented algorithm is the first to answer conjunctive queries in a description logic that admits general role inclusion axioms.