Nested constructs vs. sub−selects in SPARQL
Axel Polleres, Juan L. Reutter, Egor V. Kostylev · AMW · 2016
The issue of subqueries in SPARQL has appeared in different papers as an extension point to the original SPARQL query language. Particularly, nested CONSTRUCT in FROM clauses are a feature that has been discussed as a potential input for SPARQL 1.1 which was resolved to be left out in favour of select subqueries under the - unproven - conjecture that such subqueries can express nested construct queries. In this paper, we show that it is indeed possible to unfold nested SPARQL construct queries into subqueries in SPARQL 1.1; our transformation, however, requires an exponential blowup in the nesting depth. This suggests that nested construct queries are indeed a useful syntactic feature in SPARQL that cannot compactly be replaced by subqueries.