query: #Official website URL (P856)
  #Must have Retrieved (P813) as qualifier
  #Cannot have the url-value as reference (P854)
  SELECT DISTINCT 
  ?item ?itemLabel 
  ?websiteP856 #official website (P856)
  ?websiteRetrievedQualifier #qualifier Retrieved P813
  ?websiteRefURL #reference --> Reference url P854
  ?websiteRefDate  #reference --> Retrieved P813
  
  WHERE 
  {
    ?item (p:P31/ps:P31) wd:Q28564,wd:Q11396180;wdt:P17 wd:Q55. 
    OPTIONAL {?item p:P856 [ps:P856 ?websiteP856].} #official website (P856)
    OPTIONAL {?item p:P856 [pq:P813 ?websiteRetrievedQualifier].} #qualifier --> Retrieved P813
    OPTIONAL {?item p:P856 [prov:wasDerivedFrom [pr:P854 ?websiteRefURL]] .} #reference --> Reference url P854
    OPTIONAL {?item p:P856 [prov:wasDerivedFrom [pr:P813 ?websiteRefDate ]] .} #reference --> Retrieved P813
    
    SERVICE wikibase:label { bd:serviceParam wikibase:language "nl". }
  
  }
  ORDER BY ASC(?item)
  