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 outside databases.

  • 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.