The finance team wants to compare paid-order revenue across customer countries.
The orders table records payments by order, while customers records each customer's country. A recent re-import has left duplicate customer records in customers, although every copy of a customer contains the same country.
Prepare a country-level report showing the total amount_usd of orders with status = 'paid'. Each paid order must contribute exactly once, even if its customer appears multiple times in customers.
Paid orders whose customer_id has no matching record in customers must still contribute to revenue under an unknown (NULL) country.
Ordering: Sort by country in ascending order, with the NULL country last.