Abstract
Ontology-Mediated Query Answering (OMQA) is a well-established framework to answer queries over Knowledge Graphs (KGs), enriched with RDFS or owl. ontologies. OMQA was originally designed for Unions of Conjunctive Queries (UCQs), and based on certain answers. More recently, OMQA has been extended to SPARQL queries, but to our knowledge, none of the efforts made in this direction (either in the literature, or the so-called W3C SPARQL entailment regimes) is able to capture both certain answers for UCQs and the standard interpretation of SPARQL over a plain graph. We formalize these as requirements to be met by any semantics that aims at conciliating certain answers and SPARQL answers, and extend these with three additional requirements. Then we define two semantics that satisfies all requirements for SPARQL queries with SELECT, UNION, JOIN, and OPTIONAL. Finally, we investigate the combined complexity of query answering under these semantics over a KG enriched with a DL-Lite(R) ontology, showing that for several fragments of SPARQL, known upper-bounds for query answering over a plain KG are matched.