From Conjunctive Queries to Instance Queries in Ontology-Mediated Querying

© 2018 CEUR-WS. All rights reserved. We consider the rewritability of ontology-mediated queries (OMQs) based on UCQs into OMQs based on (certain kinds of) SPARQL queries. Our focus is on ALCI as a paradigmatic expressive DL. For rewritability into SPARQL queries that are unions of basic graph patterns, we show that the existence of a rewriting is decidable, based on a suitable characterization; for unary OMQs, this coincides with rewritability into instance queries. For SPARQL queries that additionally admit projection, we make some interesting first observations. In particular, we show that whenever there is a rewriting, then there is one that uses the same TBox as the original OMQ and only ALCI concepts from a certain finite class of such concepts. We also observe that if the TBox of the original OMQ falls into Horn-ALCI, then a rewriting always exists.

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