Question: Which river ends at  Thames Estuary and North Sea?
SPARQL:
SELECT DISTINCT ?uri WHERE { ?uri <http://dbpedia.org/ontology/riverMouth> <http://dbpedia.org/resource/Thames_Estuary> . ?uri <http://dbpedia.org/ontology/riverMouth> <http://dbpedia.org/resource/North_Sea> . ?uri <http://www.w3.org/1999/02/22-rdf-syntax-ns#type> <http://dbpedia.org/ontology/River>}