inner join (select exam_id,school_id,school_name from single_yuejuan_source.bus_exam_student group exam_id,school_id,school_name) b on a.exam_id=b.exam_id</sql>
inner join (select exam_id,school_id,school_name from single_yuejuan_source.bus_exam_student group exam_id,school_id,school_name) b on a.exam_id=b.exam_id</sql>