Ontop and R2RML
R2RML is a mapping language that can map relational tables to RDF. The mappings are written in RDF and creates a virtual triplestore that can be queried with SPARQL. The SPARQL-query is then translated into SQL over the mapped sources using the mappings.
Ontop is an implementation of R2RML, that also supports it own terser mapping language called Ontop Mappings. Ontop also supports adding an ontology together with the mappings that is used in the rewriting of SPARQL-queries. This forms the basis of an Ontology Based Data Access (OBDA) system. The ontology is typically used to integrate graphs resulting from mapping multiple sources. The ontology can also be used to introduce aditional abstraction layers.
Ontop can be used from the command line, and only needs a mapping file and a configuration file with the details for a database connection to work.