CREATE TABLE Addresses (city VARCHAR, address_id VARCHAR)
CREATE TABLE Customers (customer_address_id VARCHAR)

Which city does has most 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(*) DESC LIMIT 1
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(*) DESC NULLS LAST LIMIT 1