Question: Was U.S. president Jackson involved in a war?
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#>
PREFIX prop: <http://dbpedia.org/property/>
ASK
WHERE 
{
	?uri rdf:type yago:PresidentsOfTheUnitedStates .
        ?uri prop:name ?name .
        FILTER (regex(?name, 'Jackson')) .
        OPTIONAL {?uri onto:battle ?battle .}
        FILTER BOUND(?battle).        
}