What are the names of the cities that all customers live in?
select t1.city_town from addresses as t1 join customers as t2 on t1.address_id = t2.address_id
Okay, what about the names of the cities that all performers live in?
select distinct t1.city_town from addresses as t1 join performers as t2 on t1.address_id = t2.address_id
Of these cities, which do not also appear on the customer cities list?
select t1.city_town from addresses as t1 join performers as t2 on t1.address_id = t2.address_id except select t1.city_town from addresses as t1 join customers as t2 on t1.address_id = t2.address_id