The Virtuoso Linked Data Views meta schema is a built-in feature
of Virtuoso's SPARQL to SQL translator. It recognizes triple
patterns that refer to graphs for which an alternate representation
is declared and translates these into SQL accordingly. The main
purpose of this is evaluating SPARQL queries against existing
relational databases. There exists previous work from many parties
for rendering relational data as RDF and opening it to SPARQL
access. We can mention D2RQ, SPASQL, Squirrel RDF, DBLP and others.
The Virtuoso effort differs from these mainly in the following:
Integration with a triple store. Virtuoso can process a query
for which some triple patterns will go to local or remote
relational data and some to local physical RDF triples.
SPARQL query can be used in any place where SQL can. Database
connectivity protocols are neutral to the syntax of queries they
transmit, thus any SQL client, e.g. JDBC, ODBC or XMLA application,
can send SPARQL queries and fetch result sets. Moreover, a SQL
query may contain SPARQL subqueries and SPARQL expressions may use
SQL built-in functions and stored procedures.
Integration with SQL. Since SPARQL and SQL share the same run
time and query optimizer, the query compilation decisions are
always made with the best knowledge of the data and its location.
This is especially important when mixing triples and relational
data or when dealing with relational data distributed across many
No limits on SPARQL. It remains possible to make queries with
unspecified graph or predicate against mapped relational data, even
though these may sometimes be inefficient.
Coverage of the whole relational model. Multi-part keys etc. are
supported in all places.