ship_orders (order_id, customer_id, ship_city_override) holds orders; a customer can type a different city for one order. ship_customers (customer_id, default_city) holds each customer's saved city.
An order ships to the first of these that is available:
1. the order's own ship_city_override 2. the customer's default_city 3. 'UNKNOWN'
A city is not available when it's NULL, empty, or only spaces. Some orders belong to a customer with no row in ship_customers. Every order must appear.
Columns: order_id, ship_city. Sort by order_id.