mysql如何把字符串拆成多行

SELECT

    substring_index(substring_index(a.chain,'_',b.help_topic_id + 1),'_' ,- 1)AS ID

FROM

    (select '1_11_1223_1242' as chain) a 

JOIN mysql.help_topic b ON b.help_topic_id <

(length(a.chain) - length( replace(a.chain, '_', '')  ) + 1)