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1.5.47.How can I use sub-queries to enable literal values based joins?
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Why?
Sophisticated access to literal values via subqueries provides powerful mechanism for enhancing sparql graph patterns via dynamic literal value generation.
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How?
Use select list variables from subqueries to produce literal object values in sparql graph patterns.
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Example
Assume in the following query, where should be used a sub-query to replace ?app:
SELECT DISTINCT ?r
WHERE
{
graph ?g
{
?r nie:url ?url .
} .
?g nao:maintainedBy ?app .
?app nao:identifier "nepomukindexer" .
}
If it is not sure that ?app is the only, for e.g., the triple ?app nao:identifier "nepomukindexer" can appear in more than one graph, then the query should be changed to:
SELECT DISTINCT ?r
WHERE
{
graph ?g
{
?r nie:url ?url .
} .
?g nao:maintainedBy ?app .
FILTER (?app = (SELECT ?a WHERE { ?a nao:identifier "nepomukindexer" }))
}
or even shorter:
SELECT DISTINCT ?r
WHERE
{
graph ?g
{
?r nie:url ?url .
} .
?g nao:maintainedBy `(SELECT ?a WHERE { ?a nao:identifier "nepomukindexer" })` .
}