Question: Give me the capitals of all countries in Africa.
SPARQL query:
PREFIX onto: <http://dbpedia.org/ontology/>
PREFIX yago: <http://dbpedia.org/class/yago/>
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX rdfs: <http://www.w3.org/2000/01/rdf-schema#>
SELECT DISTINCT ?uri ?string
WHERE { 
	?states rdf:type yago:AfricanCountries .
        ?states onto:capital ?uri .
        OPTIONAL { ?uri rdfs:label ?string. FILTER (lang(?string) = 'en') }
}