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LDQL: A Query Language for the Web of Linked Data

35 Pages Posted: 9 Jul 2018 First Look: Accepted

See all articles by Olaf Hartig

Olaf Hartig

Linkoping University - Department of Computer and Information Science (IDA)

Jorge Pérez

University of Chile - Department of Computer Science

Abstract

The Web of Linked Data is composed of tons of RDF documents interlinked to each other forming a huge repository of distributed semantic data. Effectively querying this distributed data source is an important open problem in the Semantic Web area. In this paper, we propose LDQL, a declarative language to query Linked Data on the Web. One of the novelties of LDQL is that it expresses separately (i) patterns that describe the expected query result, and (ii) Web navigation paths that select the data sources to be used for computing the result. We present a formal syntax and semantics, prove equivalence rules, and study the expressiveness of the language. In particular, we show that LDQL is strictly more expressive than all the query formalisms that have been proposed previously for Linked Data on the Web. We also study some computability issues regarding LDQL. We first prove that when considering the Web of Linked Data as a fully accessible graph, the evaluation problem for LDQL can be solved in polynomial time. Nevertheless, when the limited data access capabilities of Web clients are considered, the scenario changes drastically; there are LDQL queries for which a complete execution is not possible in practice. We formally study this issue and provide a sufficient syntactic condition to avoid this problem; queries satisfying this condition are ensured to have a procedure to be effectively evaluated over the Web of Linked Data.

Suggested Citation

Hartig, Olaf and Pérez, Jorge, LDQL: A Query Language for the Web of Linked Data (2016). Journal of Web Semantics First Look. Available at SSRN: https://ssrn.com/abstract=3199269 or http://dx.doi.org/10.2139/ssrn.3199269

Olaf Hartig (Contact Author)

Linkoping University - Department of Computer and Information Science (IDA) ( email )

Linkoping, 58183
Sweden

Jorge Pérez

University of Chile - Department of Computer Science

Avenida Blanco Encalada
Santiago
Chile

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