Calculate the number of seats not yet purchased for each flight. Use three tables: playground.flights (with columns: flight_id, plane_id), playground.planes (with columns: plane_id, number_of_seats), and playground.purchases (with columns: flight_id, seat_no). Each row in purchases is unique for (flight_id, seat_no) pairs. For each flight_id, compute the free_seats as the number of seats not purchased. Order the results by flight_id in ascending order. Ensure consistency in purchases, with no records for non-existing flight_ids or seat_nos.

• flight_id int
• plane_id int

• plane_id int
• number_of_seats int

• flight_id int
• seat_no int

SELECT * FROM playground.flights LIMIT 100