mysql列转行

SELECT 
  SUBSTRING_INDEX(
    SUBSTRING_INDEX(group_id, ',', b.help_topic_id + 1), ',',- 1) 
FROM
  `group_sms_send_rule` a 
  JOIN mysql.`help_topic` b 
    ON b.help_topic_id < (
      LENGTH(a.`group_id`) - LENGTH(REPLACE(a.`group_id`, ',', '')) + 1 ) 
WHERE id = 1716 ;