PREFIX  xsd:    <http://www.w3.org/2001/XMLSchema#>
PREFIX  :       <http://example/>
PREFIX  ex:     <http://example/ns#>

# 3 optionals introducing a variable which is not constraint outside.

SELECT *
{
   ?x :p ?z  
   OPTIONAL { ?z :q1 ?w }
   OPTIONAL { ?z :q2 ?w }
   OPTIONAL { ?z :q3 ?w }
}