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%META:TOPICPARENT{name="VirtTipsAndTricksGuide"}% ---+How can I use sub-queries to enable literal value based joins? ---++What? Use of SPARQL sub-queries to enable the dynamic generation of literal values for SPO triple objects. ---++Why? Sophisticated access to literal values via sub-queries provides a powerful mechanism for enhancing SPARQL graph patterns via dynamic literal value generation. ---++How? Use select list variables from sub-queries to produce literal object values in SPARQL graph patterns as demonstrated in the example below. ---+++Example Assume the following query in which for <code>?app</code> there is only exactly one resource which has this identifier: <verbatim> SELECT DISTINCT ?r WHERE { graph ?g { ?r nie:url ?url . } . ?g nao:maintainedBy ?app . ?app nao:identifier "nepomukindexer" . } </verbatim> If one is not sure that <code>?app</code> is the only identifier (e.g., if triple <code>?app nao:identifier "nepomukindexer"</code> can appear in more than one graph), then the query can be rewritten to: <verbatim> SELECT DISTINCT ?r WHERE { graph ?g { ?r nie:url ?url . } . ?g nao:maintainedBy ?app . FILTER (?app = (SELECT ?a WHERE { ?a nao:identifier "nepomukindexer" })) } </verbatim> or even simpler: <verbatim> SELECT DISTINCT ?r WHERE { graph ?g { ?r nie:url ?url . } . ?g nao:maintainedBy `(SELECT ?a WHERE { ?a nao:identifier "nepomukindexer" })` . } </verbatim> ---+++Related * [[VirtTipsAndTricksGuide][Virtuoso Tips and Tricks Collection]]

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