mysql 获取行号 按

SELECT
   cc.camp_no,
   cc.content_id,
   IF( @camp_no <> camp_no, @rn := 0, @rn ),
   @camp_no := camp_no,
   @rn := @rn + 1 AS RowNum
 FROM grt_service.camp_content cc, (SELECT @rn := 0, @camp_no := NULL) AS t