Show all cities where at least one customer lives in but no performer lives in.
return cities ;return customers that  live in #1 ;return the  number of  #2 for each #1 ;return #1 where #3 is  at least one ;return performer who lives in #4 ;return the  number of  #5 for each #4 ;return #4 where #6 is  zero
['select', 'project', 'group', 'comparative', 'project', 'group', 'comparative']