SELECT
    ea.id,
    ea.name,
    ea.capital_balance,
    ea.total_balance,
    ea.available_balance,
    ea.cashable_balance,
    ea.market_value as old_market_value,
    mv.market_value,
    ea.total_balance + mv.market_value as total_value,
    ea.brokerage_rate,
    ea.state,
    ea.description,
    ea.created_at
FROM
    external_accounts ea,
    (
        SELECT
            c.external_account_id,
            SUM(c.amount) AS market_value
        FROM
            (
                SELECT
                    a.external_account_id,
                    a.total_count * b.price AS amount
                FROM
                    main_inventories a
                JOIN stocks b ON a.stock_code = b.code
            ) AS c
        GROUP BY
            c.external_account_id
    ) AS mv
WHERE
    ea.id = mv.external_account_id

主要是子Select那一段不知道怎么写，之所以非要一条sql写完的原因就是因为涉及到排序。