List the id of students who registered some courses and the number of their registered courses?
return students ;return courses  that  #1 registered  ;return number of  #2 for each #1 ;return #1 where #3 is  at least one ;return courses  that  #4 registered  ;return number of  #5 for each #4 ;return ids of #4 ;return #7 ,  #6
['select', 'project', 'group', 'comparative', 'project', 'group', 'project', 'union']