一种规范的方法是连接到一个子查询，该子查询查找和：

SELECT
    t1.bag_no,
    t1.product_id,
    t2.total_price
FROM myTable t1
INNER JOIN
(
    SELECT bag_no, SUM(price) AS total_price
    FROM myTable
    GROUP BY bag_no
) t2
    ON t1.bag_no = t2.bag_no
WHERE
    t1.mainproduct = 'Y';

通过mysql 8+，支持解析函数，我们可以稍微简化一下上面的查询：

WITH cte AS (
    SELECT
        t1.bag_no,
        t1.product_id,
        SUM(price) OVER (PARTITION BY t1.bag_no) total_price,
        t1.mainproduct
    FROM myTable t1
)

SELECT bag_no, product_id, total_price
FROM cte
WHERE mainproduct = 'Y';