Кирилл

SELECT a.* 
FROM test a
INNER JOIN (
	SELECT a.product_code, a.brand, minday, min(price) as minprice
	FROM test a
	INNER JOIN (
		SELECT product_code, brand, MIN(day) minday
		FROM test
		GROUP BY product_code, brand
		) b  
	ON a.product_code = b.product_code 
		AND	a.brand = b.brand 
		AND	a.day = b.minday
	GROUP BY product_code, brand, day) b
ON b.product_code = a.product_code 
	AND b.brand = a.brand 
	AND minday = a.day 
	AND minprice = a.price
WHERE по желанию