query: select ?person ?personLabel ?startyear ?endyear where
  {
    ?person p:P39 ?positionStatement . ?positionStatement ps:P39 wd:Q56824766 . 
   
    optional { ?positionStatement pq:P580 ?start . bind(year(?start) as ?startyear) } # id start year
    optional { ?positionStatement pq:P582 ?end . bind(year(?end) as ?endyear) } # id end year
    SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],en". }
  } order by ?startyear