SELECT
    industry,
    round(sum(sale_amount), 2) total_sales_amount
FROM
    (
        SELECT DISTINCT
            b.industry,
            a.sale_amount
        FROM
            sales_underline AS a
            JOIN merchants_underline AS b ON a.merchant_id = b.merchant_id
    ) AS a
GROUP BY
    industry
ORDER BY
    total_sales_amount DESC,
    industry ASC

此题ez，依旧是先表连结然后GROUP BY 就好了