OBDA

 In OBDA, a conceptual layer is
provided in the form of an ontology that defines a
shared vocabulary, models the domain, hides the struc-
ture of the data sources, and enriches incomplete data
with background knowledge. Then, queries are posed
over this high-level conceptual view, and the users
no longer need an understanding of the data sources,
the relation between them, or the encoding of the
data. Queries are translated by the OBDA system into
queries over potentially very large (usually relational
and federated) data sources. 

see Ontop: Answering SPARQL queries over relational databases 1