The database landscape has been significantly diversified during the last decade, resulting in the emergence of a variety of non-relational (also called NoSQL) databases, e.g., XML and JSON-document databases, key-value stores, and graph databases. To facilitate access to such databases and to enable data integration of non-relational data sources, we generalize the well-known ontologybased data access (OBDA) framework so as to allow for querying arbitrary databases through a mediating

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... We instantiate this framework to MongoDB, a popular JSON-document database, and implement a prototype extension of the virtual OBDA system Ontop for answering SPARQL queries over MongoDB.