Most database applications are designed according the ANSI/SPARC architecture. When it is used, a large amount of semantics of data may be lost during the transformation from the conceptual model to a logical model. As a consequence exchanging/integrating various databases or generating user interfaces for data access become difficult. Ontologies seem an interesting solution to solve these problems, since they allow making explicit the semantics of data. In this paper, we propose an ontology-based approach for designing database applications, and then, for representing explicitly the semantics of data within the database. It consists in extending the ANSI/SPARC architecture with the ontological level. Note that this extension may also be added to existing applications designed according to the ANSI/SPARC architecture, since it preserves an upward compatibility.