WITH cte AS (
SELECT PIP.purchase_id, SUM(PIP.product_count * PR.new_price) AS all_sum
FROM product_in_purchase AS PIP
RIGHT OUTER JOIN price_register AS PR ON PIP.product_id = PR.product_id
GROUP BY PIP.purchase_id
)
( SELECT * FROM cte ORDER BY all_sum ASC LIMIT 1 )
UNION
( SELECT * FROM cte ORDER BY all_sum DESC LIMIT 1 )