MySQL取序列号

SELECT @rownum := @rownum +1 as xh,a.* FROM (select @rownum :=0) r,requisition_base a ORDER BY id desc;

        SELECT
            @rownum := @rownum + 1 id,
            m.*
            FROM
            (SELECT @rownum := 0) r,
            (
            SELECT
            ro.order_id,
            ro.requisition_id,
            ob.order_code,
            ob.user_id,
            ob.user_name,
            ob.id_card_value,
            ob.mobile,
            ob.payee_bank_name,
            ob.payee_bank_account,
            ob.order_money,
            ob.pay_status,
            ob.in_order_flow_id,
            ob.failure_reasons,
            (
            CASE
            WHEN ob.pay_status = 0 THEN
            '待支付'
            WHEN ob.pay_status = 1 THEN
            '支付成功'
            WHEN ob.pay_status = 2 THEN
            '支付中'
            WHEN ob.pay_status = 3 THEN
            '支付失败'
            ELSE
            ''
            END
            ) pay_status_name
        FROM
            requisition_order ro
        LEFT JOIN order_base ob ON ob.id = ro.order_id
        <where>
            AND ro.relation_status = 1
            <if test="requisitionId != null ">
                AND ro.requisition_id = #{requisitionId}
            </if>
        </where>
        ORDER BY ob.create_time DESC
        ) m