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Subqueries are a powerful feature which allows enforcing reuse, composition, rewriting and optimization in a query language. In this paper the authors perform a comprehensive study of the incorporation of subqueries into SPARQL. They consider the many possible choices as suggested by the experience of similar languages, as well as features that developers are incorporating and/or experimenting with. Their study includes the discussion in the SPARQL case of common design issues concerning subqueries (i.e., query correlation, scope of datasets, and optimization), show the interplay among them, and study their implications in terms of expressive power.
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