The RDF Query Language (RQL)

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Although voluminous RDF/S graphs have already appeared (e.g. in digital libraries and museums), expressive declarative languages for querying both RDF descriptions and schemas are still in their infancy. We pay particular attention to the declarative nature of RDF/S manipulation languages for the old and good reasons: logical/physical data independence, efficient query/update optimization and transaction management.

RQL is still the only declarative language for querying both explicitly stated triples of RDF/S graphs and inferred ones by transitivity of subsumption and type relationships. It is a typed, functional language (a la ODMG-OQL with limited recursion which relies on a formal model for RDF/S graphs permitting the interpretation of instances by means of one or more schema vocabularies. RQL adapts the functionality of semistructured/ XML query languages to the peculiarities of the RDF/S data model but, foremost, it integrates smoothly RDF/S reasoning with querying (called /semantics-aware querying/). To this end, RQL provides sophisticated /pattern matching facilities/ under the form of generalized path expressions (GPEs) featuring variables on both labels for nodes (i.e., classes) and edges (i.e., properties). Given an RDF/S graph G, an RQL query consists of one or several class and property patterns which are matched against schema and/or instance triples of G, and the variable bindings obtained from this matching are further processed (filtered, projected or grouped) to form the answer. It is worth noticing that RQL queries can be arbitrary composed while their semantics is formalized using standard database machinery.

Main Features


The RQL is currently available under the RDFSuite license. It has both a runtime and a source distribution. To install and run RQL, C++ (gcc-2.95.1 or higher) and PostgreSQL (v7.3 or higher) DBMS are required. RQL v2.1 also includes a 'configure' script for easier installation and customization on different platforms. See more details at Installation Instructions. An alternative implementation of an RQL subset is available by aidministrator.


The author of the software is Greg Karvounarakis (CS Department - University of Crete and ICS-FORTH - Greece).


Copyright ICS-FORTH (Institute of Computer Science - Foundation of Research Technology Hellas - Greece)


For feedback and questions please contact:

Greg Karvounarakis
Vassilis Christophides

Last modified: Oct 9, 2008