SELECT
    其他字段,
    SUBSTRING_INDEX( SUBSTRING_INDEX( a.'逗号分隔字段', ',', b.help_topic_id + 1 ), ',',- 1 ) AS '分拆后字段' 
FROM
    `表` AS a
    JOIN mysql.help_topic AS b ON b.help_topic_id < ( length( a.'逗号分隔字段' ) - length( REPLACE ( a.'逗号分隔字段', ',', '' ) ) + 1 )
; 

//mysql.help_topic 是系统表，有连续序列