WITH t1 AS (
    SELECT a.item_id,
           a.sales_num,
           a.sales_price,
           b.tag_price
    FROM sales_tb a
    LEFT JOIN product_tb b
    on a.item_id = b.item_id
),
t2 AS (
    SELECT
        SUM(sales_price) AS after_price,
        SUM(sales_num * tag_price) AS orign_price
    FROM t1
)SELECT ROUND(
    after_price / orign_price * 100
    ,2) AS `discount_rate(%)`
    FROM t2;