UNION — disjoint pattern alternatives

{ A } UNION { B } is a real set union of solution mappings. Variables only present in one branch are null in solutions from the other.

What it does

A SPARQL UNION joins the solutions of two patterns into one output stream. Each branch is evaluated independently; the combined result preserves variables from both, with null where a variable wasn't bound on the branch that produced a given row.

Why you'd use it

• Ontologists — express "either of these shapes match" without writing two queries. Vital for traversing alternative property names or class subdivisions in heterogeneous data.
• Data scientists — combine views over different sub-graphs in one SPARQL call.

Example

SELECT * FROM pgrdf.sparql(
  'PREFIX foaf: <http://xmlns.com/foaf/0.1/>
   SELECT ?s ?n ?m
     WHERE { { ?s foaf:name ?n }
             UNION
             { ?s foaf:mbox ?m } }');

Rows from the first branch carry ?n but ?m = null; rows from the second branch carry ?m but ?n = null. The shared ?s is populated by whichever branch produced the solution.

Tests

• tests/w3c-sparql/03-union-disjoint/ — W3C §18.2.4 conformance.
• tests/regression/sql/37-sparql-union.sql