Abstract
We address the problem of query answering with ontologies over databases. We consider first-order ontology systems playing the role of a conceptual model of a database represented as a classical finite relational store, either with an open world or a closed world reading. Queries over the conceptual signature are reformulated into queries over the database signature, so to get the same answer directly via SQL relational database technology. We consider two distinct approaches to reformulation, perfect and exact reformulation. We discuss advantages and disadvantages of each of the two approaches, and we report on some significant results appeared in the literature.