A Review of Answering Queries over Ontologies Based on Databases

Ontologies, as a standard (W3C recommendation) for representing knowledge in the Semantic Web, have been employed in many application domains. Currently, real ontologies tend to become very large to huge. Thus, one problem is considered that has arisen from practical needs: namely, efficient querying of ontologies. To this end, there are today many proposals for answering queries over ontologies, and until now the literature on querying of ontologies has been flourishing. In particular, on the basis of the efficient and mature techniques of databases, which are useful for querying ontologies. To investigate querying of ontologies and more importantly identifying the direction of querying of ontologies based on databases, in this chapter, we aim at providing a brief review of answering queries over ontologies based on databases. Some query techniques, their classifications and the directions for future research, are introduced. Other query formalisms over ontologies that are not related to databases are not covered here.