What is the count of states with college students playing in the mid position but not as goalies?
return college students ;return #1 that are playing in  the  mid position ;return #1 that are playing in  the  goalie position ;return states of #2 ;return states of both #2 and  #3 ;return #4 besides  #5 ;return number of  #6
['select', 'filter', 'filter', 'project', 'intersection', 'discard', 'aggregate']