select  staff_id  ,sum(grade_id)/(select count(*) from tb_order_history group by staff_id )                from tb_order_history ,tb_feedback                where tb_order_history.feedback_id = tb_feedback.feedback_id group by tb_order_history.staff_id```  現(xiàn)在因為子查詢返回多行，所以報錯。我想除數(shù)位置的子查詢按staff_id 分組 統(tǒng)計返回的count（*）和父查詢的staff_id 對應(yīng)，該怎么辦，求大神解答，多謝??！```