Geospatial Ontology-based Data Access

11.1 Ontology-based Data Access The OBDA paradigm [Xiao et al. 2018] proposes the creation of virtual RDF graphs on top of relational databases using ontologies and mappings. Given a database schema S, an ontology O, and a set of mappings M, an OBDA specification is a triple P = (O, M, S). Then, an OBDA instance (P, D) is defined given the OBDA specifica­ tion P and the database D that follows the database schema S. Mappings encode how relational data get mapped into RDF terms. A virtual RDF graph VGM,D of the database instance D is produced if we apply the mappings M to D. Then, if [[Q]](P,D) is the result of the evaluation of the SPARQL query Q over the OBDA instance (P, D), this is equivalent to [[Q]](P,VG(M,D)). The W3C standard language for encoding mappings of relational data is R2RML [Das et al. 2012], which was briefly introduced in Chapter 9. As an example, we can map a relational table named Student with columns id and name into RDF with the following R2RML mapping: