Find the student ID and middle name for all the students with at most two enrollments.
return students ;return enrollments of  students ;return number of #2 for each #1 ;return #1 where  #3 is  at most two ;return student ID of  #4 ;return middle names of  #4 ;return #5 ,   #6
['select', 'select', 'group', 'comparative', 'project', 'project', 'union']