Which city has the least number of customers?
SELECT T2.city FROM Customers AS T1 JOIN Addresses AS T2 ON T1.customer_address_id  =  T2.address_id GROUP BY T2.city ORDER BY count ( * )   LIMIT 1
least number of customers refers to MIN(count(customer_id)); city related to customers through their address refers to JOIN on customer_address_id = address_id;