List all the cities customers live 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
Group by cities and count how many customers live there.
select count(*) 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 group by t3.city
Which city has the most customers?
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 group by t3.city order by count(*) desc limit 1