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

SELECT (?s1 AS ?subset) (?s2 AS ?superset)
WHERE
{
    # All pairs of sets
    ?s2 rdf:type :Set .
    ?s1 rdf:type :Set .

    MINUS {
        ?s1 rdf:type :Set .
        ?s2 rdf:type :Set .
        # Assumes ?s1 has at least one member 
        ?s1 :member ?x .
        # If we want to exclude A as a subset of A.
        # This is not perfect as "?s1 = ?s2" is not a
        # contents based comparison.
        FILTER ( ?s1 = ?s2 || NOT EXISTS { ?s2 :member ?x . } )
    }
    MINUS {
        # If we don't want the empty set being a subset of itself.
        ?s1 rdf:type :Set .
        ?s2 rdf:type :Set .
        # Choose the pair (empty set, empty set)
        FILTER ( NOT EXISTS { ?s1 :member ?y . } )
        FILTER ( NOT EXISTS { ?s2 :member ?y . } )
    }
}