FeDeRate [FED] provides a mapping between RDF queries and SQL queries over conventional relational databases. SPASQL provides similar functionality, but eliminates the query rewriting phase by parsing the SPARQL [SPARQL] directly in the database server. This approach is efficient, and allows applications linked to currently deployed MySQL client libraries to execute SPARQL queries and get the results back through the conventional MySQL protocol.
This looks like a nice, pragmatic approach to solving the legacy SQL data problem. There's some interesting discussion about the mismatch between UNION in SQL and SPARQL.