For each customer, find the city he or she lives in.
select t3.city from customers as t1 join customer_addresses as t2 on t1.customer_id = t2.customer_id join addresses as t3 on t2.address_id = t3.address_id
What are all the distinct cities customers live in?
select distinct t3.city from customers as t1 join customer_addresses as t2 on t1.customer_id = t2.customer_id join addresses as t3 on t2.address_id = t3.address_id
What are the cities no customers live in?
select city from addresses where city not in ( select distinct t3.city from customers as t1 join customer_addresses as t2 on t1.customer_id = t2.customer_id join addresses as t3 on t2.address_id = t3.address_id)