An RDF to RDB Gateway

RDF is a language for expressing Directed Labeled Graphs (DLGs) using URIs (a superset of URLs) as node and arc identifiers. 

The link http://www.w3.org/2003/01/21-RDF-RDB-access/ discusses Federate, a gateway that permits RDF access to data in an RDB.  At present, Federate works with two reasoning agents, algae [1], and cwm [2]. 

Federate provides RDF agents with the ability to query a relational database with an application-specific schema. Querying RDF stored as Statements incurs the cost of an additional join per restriction condition (WHERE or JOIN ON clause in SQL). In general, properly normalized relational databases do not need a join for unique properties, ie, properties for which there is one and only one slot. Federate provides RDF data with the speed, compactness, and integrity constraints of conventional relational databases.