  Ontology-Mediated Query Answering (OMQA) is a well-established framework to
answer queries over an RDFS or OWL Knowledge Base (KB). 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 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 aiming at
conciliating certain answers and SPARQL answers, and define three additional
requirements, which generalize to KBs some basic properties of SPARQL answers.
Then we show that a semantics can be defined that satisfies all requirements
for SPARQL queries with SELECT, UNION, and OPTIONAL, and for DLs with the
canonical model property. We also investigate combined complexity for query
answering under such a semantics over DL-Lite R KBs. In particular, we show for
different fragments of SPARQL that known upper-bounds for query answering over
a plain graph are matched.
