query: # units that are not proper items
  SELECT ?value ?unit ?count WITH {
    SELECT ?unit (COUNT(*) AS ?count) (SAMPLE(?value) AS ?value) WHERE {
      ?value wikibase:quantityUnit ?unit.
    }
    GROUP BY ?unit
  } AS %units WHERE {
    INCLUDE %units.
    MINUS { ?unit wikibase:statements ?statements. }
  }