Ioanna Koffina Giorgos Serfiotis Vassilis Christophides Val Tannen Mediating RDF/S Queries to Relational and XML Sources. 68-91 2006 2 Int. J. Semantic Web Inf. Syst. 4 http://www.igi-pub.com/articles/details.asp?ID=6648 db/journals/ijswis/ijswis2.html#KoffinaSCT06