Question: What is the longest river?
SPARQL query:
PREFIX onto: <http://dbpedia.org/ontology/>
PREFIX prop: <http://dbpedia.org/property/>
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 
{
        ?uri rdf:type onto:River .
        ?uri prop:length ?x .
        OPTIONAL { ?uri rdfs:label ?string. FILTER (lang(?string) = 'en') }
}
ORDER BY DESC(?x) OFFSET 0 LIMIT 1