What are the names of the directors who made exactly one movie excluding director NULL?
return directors ;return director NULL ;return #1 besides #2 ;return movies that  #3 made ;return the  number of  #4 for each #3 ;return #3 where #5 is  one ;return the  names of #6
['select', 'select', 'discard', 'project', 'group', 'comparative', 'project']