mysql 按逗号拆分多行数据

SELECT
    a.CMAGREEMENT_NO,
    a.ATTACHMENT,
    substring_index(
        substring_index(
            a.ATTACHMENT,
            ',',
            b.help_topic_id + 1
        ),
        ',' ,- 1
    ) AS attachment_id
FROM
    bs_cmagreement a
JOIN mysql.help_topic b ON b.help_topic_id < (
    length(a.ATTACHMENT) - length(
        REPLACE (a.ATTACHMENT, ',', '')
    ) + 1
) and a.IS_DEL = 0 
-- and a.ATTACHMENT is not null

ps: bs_cmagreement.ATTACHMENT 是需要拆分的字段