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ISWC '09 Proceedings of the 8th International Semantic Web Conference
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JELIA'10 Proceedings of the 12th European conference on Logics in artificial intelligence
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RW'13 Proceedings of the 9th international conference on Reasoning Web: semantic technologies for intelligent data access
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This chapter accompanies the lecture on SPARQL with entailment regimes at the 7th Reasoning Web Summer School in Galway, Ireland, 2011. SPARQL is a query language and protocol for data specified in the Resource Description Format (RDF). The basic evaluation mechanism for SPARQL queries is based on subgraph matching. The query criteria are given in the form of RDF triples possibly with variables in place of the subject, object, or predicate of a triple, called basic graph patterns. Each instantiation of the variables that yields a subgraph of the queried RDF graph constitutes a solution. The query language further contains capabilities for querying for optional basic graph patterns, alternative graph patterns etc. We first introduce the main features of SPARQL as a query language. In order to define the semantics of a query, we show how a query can be translated to an abstract query, which can then be evaluated according to SPARQL's query evaluation mechanism. Apart from the features of SPARQL 1.0, we also briefly introduce the new features of SPARQL 1.1, which is currently being developed by the Data Access Working Group of the World Wide Web Consortium. In the second part of these notes, we introduce SPARQL's extension point for basic graph pattern matching. We illustrate how this extension point can be used to define a semantics for basic graph pattern evaluation based on more elaborate semantics such as RDF Schema (RDFS) entailment or OWL entailment. This allows for solutions to a query that implicitly follow from an RDF graph, but which are not necessarily explicitly present. We illustrate what constitutes an extension point and how problems that arise from using a semantic entailment relation can be addressed. We first introduce SPARQL in combination with the RDFS entailment relation and then move on to the more expressive Web Ontology Language OWL. We cover OWL's Direct Semantics, which is based on Description Logics, and the RDF-Based Semantics, which is an extension of the RDFS semantics. For the RDF-Based Semantics we mainly focus on the OWL 2 RL profile, which allows for an efficient implementation using rule engines. We assume that readers have a basic knowledge of RDF and Turtle, which we use in examples. For the OWL parts, we assume some background in OWL or Description Logics (see lecture notes Foundations of Description Logics). The examples for the OWL part are given in Turtle, OWL's functional-style syntax and Description Logics syntax. Although the inferences that are relevant for the example queries are explained, a basic idea about OWL's modeling constructs and their semantics are certainly helpful.