Question: Who are the presidents of the United States?
SPARQL query:
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX yago: <http://dbpedia.org/class/yago/>
PREFIX onto: <http://dbpedia.org/ontology/>
PREFIX res: <http://dbpedia.org/resource/>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
PREFIX prop: <http://dbpedia.org/property/>
SELECT DISTINCT ?uri ?string
WHERE 
{
        {
          ?uri rdf:type yago:PresidentsOfTheUnitedStates.         
        }
        UNION
        { 
          ?uri rdf:type onto:President.
          ?uri prop:title res:President_of_the_United_States.
        }
        OPTIONAL {?uri rdfs:label ?string. FILTER (lang(?string) = 'en') }
}