PREFIX : <http://example/>
PREFIX rdf:    <http://www.w3.org/1999/02/22-rdf-syntax-ns#>

SELECT ?p
{
    ?x rdf:type :X .
    ?x :p ?p .
    ?x :q ?q .
    ?x :n ?n .
}
GROUP BY ?p (?q = "a" && ?n = 2)