mysql逗号分隔的字符串转换为行

SELECT a.id,
substring_index( substring_index( a.code, ',', b.help_topic_id + 1 ), ',',- 1 ) age,a.year
FROM t a
JOIN mysql.help_topic b ON b.help_topic_id < ( length( a.code ) - length( REPLACE ( a.code, ',', '' ) ) + 1 ) 
ORDER BY a.id