Context. Stack of orders, but the customer table doesn't have every customer. Keep all orders; fill missing names with "Unknown".
Your task. Write attach_or_unknown(orders, customers) that left-joins on customer_id and replaces missing names with "Unknown".
Example. orders for customers 10, 20, 30; customers has only 10, 20 → 3 rows, with name="Unknown" for the order with customer 30.
Notes. Same row order as orders. Every order matched → no "Unknown" substitution.