What are the id of students who registered courses or attended courses?
return students ;return registered courses of #1 ;return number of  #2 for each #1 ;return #1 where #3 is higher than zero ;return attended courses of #1 ;return number of  #5 for each #1 ;return #1 where #6 is higher than zero ;return #4 or  #7 ;return ids of #8
['select', 'project', 'group', 'comparative', 'project', 'group', 'comparative', 'union', 'project']