Customers who out-spend the average customer

The loyalty team wants to reward customers whose total spend is above the average total spend of all customers.

From purchases, return customer_name and total_spent for every customer whose total spend is greater than the average of all customer totals. Show the biggest spender first.

1. Step 1: First build per-customer totals in a subquery, then average those totals. `AVG(amount)` over raw rows would give the average purchase, which is a different number.
2. Step 2: The average of customer totals here is 5925 (35550 / 6).
3. Step 3: The outer query groups again and uses `HAVING SUM(amount) > (subquery)` to keep only the customers above it.

SELECT customer_name,
       SUM(amount) AS total_spent
FROM purchases
GROUP BY customer_name
HAVING SUM(amount) > (
    SELECT AVG(customer_total)
    FROM (SELECT SUM(amount) AS customer_total
          FROM purchases
          GROUP BY customer_name) AS totals
)
ORDER BY total_spent DESC;