Truy vấn con (Subqueries)

SELECT pt.name,
    pt.category,
    SUM(od.quantity) AS total_orders
FROM pizza_type pt
JOIN pizzas p
    ON pt.pizza_type_id = p.pizza_type_id
JOIN order_details od
    ON p.pizza_id = od.pizza_id
GROUP BY ALL
HAVING SUM(od.quantity) < (
  -- Calculate AVG of total_quantity
  SELECT ___(___)
  FROM (
    -- Calculate total_quantity
    SELECT ___(___.___) AS total_quantity
    FROM pizzas p
    JOIN order_details od 
    	ON p.pizza_id = od.pizza_id
    GROUP BY p.pizza_id
    -- Alias as subquery
  ) ___
)