In this paper we present Ontop, an open-source Ontology Based Data Access (OBDA) system that allows for querying
relational data sources through a conceptual representation of the domain of interest, provided in terms of an ontology, to which
the data sources are mapped. Key features of Ontop are its solid theoretical foundations, a virtual approach to OBDA that avoids
materializing triples and that is implemented through query rewriting techniques, extensive optimizations exploiting all elements
of the OBDA architecture, its compliance to all relevant W3C recommendations (including SPARQL queries, R2RML mappings,
and OWL 2 QL and RDFS ontologies), and its support for all major relational databases
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