Question: What has Carl Sagan written his books about?
SPARQL query:
PREFIX rdf: <http://www.w3.org/1999/02/22-rdf-syntax-ns#>
PREFIX prop: <http://dbpedia.org/property/>
PREFIX onto: <http://dbpedia.org/ontology/>
SELECT DISTINCT ?uri WHERE { ?x onto:author <http://dbpedia.org/resource/Carl_Sagan> . ?x prop:subject ?uri  . ?x rdf:type onto:Book}