RQL

TLDR

Semantic Web applications rely on large volumes of metadata, and while RDF supports metadata creation and exchange, expressive query languages for both RDF data and schemas are still lacking. The paper introduces RQL, a new RDF query language designed to fill this gap. RQL is a typed functional language built on a formal model of directed labeled graphs that enables uniform querying of both RDF resource descriptions and schemas, adapting semistructured/XML query language features. The authors illustrate RQL’s syntax, semantics, and typing with example queries and demonstrate that its persistent RDF store performs efficiently.

Abstract

Real-scale Semantic Web applications, such as Knowledge Portals and E-Marketplaces, require the management of large volumes of metadata, i.e., information describing the available Web content and services. Better knowledge about their meaning, usage, accessibility or quality will considerably facilitate an automated processing of Web resources. The Resource Description Framework (RDF) enables the creation and exchange of metadata as normal Web data. Although voluminous RDF descriptions are already appearing, sufficiently expressive declarative languages for querying both RDF descriptions and schemas are still missing. In this paper, we propose a new RDF query language called RQL. It is a typed functional language (a la OQL) and relies on a formal model for directed labeled graphs permitting the interpretation of superimposed resource descriptions by means of one or more RDF schemas. RQL adapts the functionality of semistructured/XML query languages to the peculiarities of RDF but, foremost, it enables to uniformly query both resource descriptions and schemas. We illustrate the RQL syntax, semantics and typing system by means of a set of example queries and report on the performance of our persistent RDF Store employed by the RQL interpreter.