There has been many research in the domain of mapping relational data from databases like MySQL or Oracle to RDF data.

• D2R Server - http://www4.wiwiss.fu-berlin.de/bizer/d2r-server/ 
  Publishing information about the Web-based Systems Group @ Freie Universität Berlin. It can be accessed using (1) your plain old web browser (2) Semantic Web browsers (3) SPARQL query clients.
• Virtuoso - http://virtuoso.openlinksw.com/
  Virtuoso is an innovative enterprise grade server that cost-effectively delivers an unrivaled platform for Data Access, Integration and Management.