prefix ex: <http://example.org/survey-sample/ontology/>
prefix acm: <http://daml.umbc.edu/ontologies/topic-ont#>
prefix rdfs: <http://www.w3.org/2000/01/rdf-schema#>

SELECT ?title 
WHERE  
     { { [] a acm:Topic; rdfs:label ?title }
     UNION
      { [] ex:title ?title }}