g1:
 a p b
 a q p

q1:
> > select ?x ?y where {
> > ?x p ?y OPTIONAL { ?y q ?z}
> > filter( ! exists {?y q ?z })

would return {?x/a ?y/b}, whereas

q2:
> > select ?x ?y where {
> > ?x p ?y OPTIONAL { ?y q ?z}
> > filter( ! exists {?v q ?z FILTER(?y != ?v) })
would return the empty solution set {}

on graph

g2:
 a p b
 b q p