Count Customers Per Spend Tier

Assign every customer a tier from their total order amount: 'gold' (1500 or more), 'silver' (1000 to 1499), 'bronze' (more than 0, under 1000) and 'none' (no orders at all). Return each tier and the number of customers in it as customers, ordered gold, silver, bronze, none. Customers without orders must still be counted.