message: Update unique-orders-and-customers-per-month.sql
diff: # Time: O(n)
  # Space: O(n)
  -SELECT LEFT(order_date, 7) month, COUNT(DISTINCT order_id) order_count,
  +SELECT LEFT(order_date, 7) month,
  + COUNT(DISTINCT order_id) order_count,
  COUNT(DISTINCT customer_id) customer_count
  FROM orders
  WHERE invoice > 20
  