Question: How many party leaders are there whose parties are headquartered in Berlin?
SPARQL:
SELECT DISTINCT COUNT(?uri) WHERE { ?x <http://dbpedia.org/ontology/headquarter> <http://dbpedia.org/resource/Berlin> . ?x <http://dbpedia.org/ontology/leader> ?uri  . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/Politician>}