Artem Chebotko Shiyong Lu Mustafa Atay Farshad Fotouhi Efficient Processing of RDF Queries with Nested Optional Graph Patterns in an RDBMS. 1-30 2008 4 Int. J. Semantic Web Inf. Syst. 4 http://www.igi-pub.com/articles/details.asp?ID=34304 db/journals/ijswis/ijswis4.html#ChebotkoLAF08