SQL to RDF access ?

Hi,

I am wondering if some of you knows of some tool that can translate (a
subset) of SQL to SPARQL (perhaps requiring some constraints on the RDF
representation).
In principle it should be simple: classes can appear as tables, URIs as
IDs, datatype properties as columns and object properties as Fkeys.
Is there something implementing this translation available, that some of
you know ? One current option (I think) is via Oracle, but I am wondering
if there is something like-weight.

As of why I am interested in it... it's curious: I may have an RDF graph
representing a unified set of sources (some of which native in RDF, some of
which virtualized from SQL).
Still people like to query the sql sources in sql, just because they know
it better. But like this, they miss the whole integration (and a more
flexible data model).

best,
Andrea