Subqueries

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Subqueries are a way to embed SPARQL queries within other queries, normally to achieve results which cannot otherwise be achieved, such as limiting the number of results from some sub-expression within the query.

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Due to the bottom-up nature of SPARQL query evaluation, the subqueries are evaluated logically first, and the results are projected up to the outer query.

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Note that only variables projected out of the subquery will be visible, or in scope, to the outer query.

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Example

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Data:

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@prefix : <http://people.example/> .\n\n:alice :name \"Alice\", \"Alice Foo\", \"A. Foo\" .\n:alice :knows :bob, :carol .\n:bob :name \"Bob\", \"Bob Bar\", \"B. Bar\" .\n:carol :name \"Carol\", \"Carol Baz\", \"C. Baz\" .\n

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Return a name (the one with the lowest sort order) for all the people that know Alice and have a name.

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Query:

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PREFIX : <http://people.example/>\nPREFIX : <http://people.example/>\nSELECT ?y ?minName\nWHERE {\n  :alice :knows ?y .\n  {\n    SELECT ?y (MIN(?name) AS ?minName)\n    WHERE {\n      ?y :name ?name .\n    } GROUP BY ?y\n  }\n}\n

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Results:

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y	minName
:bob	\"B. Bar\"
:carol	\"C. Baz\"

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This result is achieved by first evaluating the inner query:

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SELECT ?y (MIN(?name) AS ?minName)\nWHERE {\n  ?y :name ?name .\n} GROUP BY ?y\n

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This produces the following solution sequence:

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y	minName
:alice	\"A. Foo\"
:bob	\"B. Bar\"
:carol	\"C. Baz\"

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Which is joined with the results of the outer query:

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y
:bob
:carol