query: SELECT ?item ?itemLabel ?RKD_url ?andereCollecties WHERE {
    ?item wdt:P6379 wd:Q64159711.
    OPTIONAL { ?item wdt:P650 ?RKDid. }
    BIND(IRI(CONCAT('https://rkd.nl/nl/explore/artists/',?RKDid)) AS ?RKD_url)
      ?item wdt:P6379 ?collecties
    BIND(?collecties =  wd:Q64159711 AS ?eigencollectie).
    BIND(IF(?eigencollectie,"", ?collecties) AS ?andereCollecties).
  }