In this paper, we further develop a proposed layered approach for the Semantic Web. Our objective is to build a specific solution to the problem of providing data interoperability among different databases, so as to allow for schematic data integration. In particular, we solve the problem of translating queries on a database schema into queries on another database schema, using their relationship with an ontology. We use RDF Schema to model the databases and the ontology. A common vocabulary expresses the mappings between each database schema and the ontology.