select
    round((sum(sales_price) / sum(tag_amt)) * 100, 2) as 'discount_rate(%'
from
    (
        select
            s.item_id,
            sales_num * p.tag_price as tag_amt,
            s.sales_price
        from
            sales_tb s
            left join product_tb p on s.item_id = p.item_id
    ) t