What is the city with the most customers?
the most customers refers to MAX(COUNT(customer_id))
SELECT T.city FROM ( SELECT T1.city, COUNT(T3.customer_id) AS num FROM city AS T1 INNER JOIN address AS T2 ON T2.city_id = T1.city_id INNER JOIN customer AS T3 ON T2.address_id = T3.address_id GROUP BY T1.city ) AS T ORDER BY T.num DESC LIMIT 1