CREATE TABLE lessons (lesson_id NUMBER, customer_id NUMBER, lesson_status_code CLOB, staff_id NUMBER, vehicle_id NUMBER, lesson_date TIME, lesson_time CLOB, price NUMBER)
CREATE TABLE customer_payments (customer_id NUMBER, datetime_payment TIME, payment_method_code CLOB, amount_payment NUMBER)
CREATE TABLE staff (staff_id NUMBER, staff_address_id NUMBER, nickname CLOB, first_name CLOB, middle_name CLOB, last_name CLOB, date_of_birth TIME, date_joined_staff TIME, date_left_staff TIME)
CREATE TABLE addresses (address_id NUMBER, line_1_number_building CLOB, city CLOB, zip_postcode CLOB, state_province_county CLOB, country CLOB)
CREATE TABLE vehicles (vehicle_id NUMBER, vehicle_details CLOB)
CREATE TABLE customers (customer_id NUMBER, customer_address_id NUMBER, customer_status_code CLOB, date_became_customer TIME, date_of_birth TIME, first_name CLOB, last_name CLOB, amount_outstanding NUMBER, email_address CLOB, phone_number CLOB, cell_mobile_phone_number CLOB)

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" "T1" JOIN "addresses" "T2" ON "T1"."customer_address_id" = "T2"."address_id" GROUP BY "T2"."city" ORDER BY COUNT(*) DESC FETCH FIRST 1 ROWS ONLY