The use of semantic knowledge in its various forms has become an important aspect in managing data in database and information systems. In the form of integrity constraints, it has been used intensively in query optimization for some time. Similarly, data integration techniques have utilized semantic knowledge to handle heterogeneity for query processing on distributed information sources in a graceful manner. Recently, ontologies have become a ”buzz word” for the semantic web and semantic data processing. In fact, they play a central role in facilitating the exchange of data between the several sources. In this paper, we present a new approach using ontology knowledge for query processing within a single relational database to extend the result of a query in a semantically meaningful way. We describe how an ontology can be effectively exploited to rewrite a user query into another query such that the new query provides additional meaningful results that satisfy the intention of ...