Show me all the cities.
select city_town from addresses
Which ones of them has at least one customer?
select t1.city_town from addresses as t1 join customers as t2 on t1.address_id = t2.address_id
Among those cities, show ones that has no performer.
select t1.city_town from addresses as t1 join customers as t2 on t1.address_id = t2.address_id except select t1.city_town from addresses as t1 join performers as t2 on t1.address_id = t2.address_id