question: Show all cities where at least one customer lives in but no performer lives in.
#1 return cities 
#2 return customers that  live in #1 
#3 return the  number of  #2 for each #1 
#4 return #1 where #3 is  at least one 
#5 return performer who lives in #4 
#6 return the  number of  #5 for each #4 
#7 return #4 where #6 is  zero