The need to answer queries over many possible worlds arises in many different settings such as data integration/exchange or in ontology based query answering. The generally agreed approach to query answering in such settings is to compute the so-called certain answers, i.e., answers that one gets by answering the query over any possible world.
In this talk, we study the problem of computing certain answers of well-designed SPARQL queries under OWL2-QL entailment. The main challenge comes from the non-monotonicity of the OPTIONAL operator. We thus first define an intuitive semantics of certain answers based on the subsumption relation. To actually compute the certain answers of well-designed SPARQL queries under OWL2-QL entailment, we extend the rewriting-based approach of Calvanese et al. to evaluating conjunctive queries w.r.t. DL-Lite.