mysql如何将一行变成多行

SELECT

    substring_index(substring_index( a.rn,',',b.help_topic_id + 1    ),  ',' ,- 1    ) AS rn

FROM

    (select SUBSTR(GROUP_CONCAT( REPEAT(CONCAT(number,','),fre) SEPARATOR ''),1,LENGTH(GROUP_CONCAT( REPEAT(CONCAT(number,','),fre) SEPARATOR ''))-1) as rn  from numbers ) a 

JOIN mysql.help_topic b ON b.help_topic_id <

(length(a.rn) - length( replace(a.rn, ',', '')  ) + 1)