RDQL has been implemented in a number of RDF systems for extracting information from RDF graphs. An RDQL consists of a graph pattern, expressed as a list of triple patterns. Each triple pattern is comprised of named variables and RDF values (URIs and literals). An RDQL query can additionally have a set of constraints on the values of those variables, and a list of the variables required in the answer set. An RDQL query treats an RDF graph purely as data. If the implementation of that graph provides inferencing to appear as "virtual triples”, then an RDQL will include those triples as possible matches in triple patterns. RDQL makes no distinction between inferred triples and ground triples. RDQL was first released in Jena 1.2.0. The following systems are known to provide RDQL: Jena, RDFStore, Sesame, PHP XML Classes, 3Store, RAP - RDF API for PHP. In addition, RDQL is one language used for remote query by the Joseki RDF Server.