Question: Through which countries does the Yenisei river flow?
SPARQL query:
PREFIX res: <http://dbpedia.org/resource/>
PREFIX prop: <http://dbpedia.org/property/>
SELECT DISTINCT ?uri ?string
WHERE 
{
        res:Yenisei_River prop:country ?string .
        OPTIONAL { ?uri prop:commonName ?name . }
        FILTER regex(?name,?string) .
        OPTIONAL { ?uri prop:dateEnd ?date . }
        FILTER (!BOUND(?date))
}