The database community is on a constant quest for better integration of data management and knowledge management. Recently, with the increasing use of ontology in various applications, the quest has become more concrete and urgent. However, manipulating knowledge along with relational data in DBMSs is not a trivial undertaking. In this paper, we introduce a novel, unified framework for managing data and domain knowledge. We provide the user with a virtual view that unifies the data, the domain knowledge and the knowledge inferable from the data using the domain knowledge. Because the virtual view is in the relational format, users can query the data and the knowledge in a seamlessly integrated manner. To facilitate knowledge representation and inferencing within the database engine, our approach leverages XML support in hybrid relational-XML DBMSs (e.g., Microsoft SQL Server & IBM DB2 9 PureXML). We provide a query rewriting mechanism to bridge the difference between logical and p...