将group_concat反转回列

value	num
1	1,2,3,4,1

此种结构 将 num 转为一列一列

SELECT DISTINCT
	a.`value`,substring_index(substring_index( a.`num`, ',', b.help_topic_id + 1 ), ',',- 1 ) as result
FROM
	test a
	JOIN mysql.help_topic b ON b.help_topic_id < (
	length( a.`num` ) - length(
	REPLACE ( a.`num` , ',', '' ))+ 1);

变成

value	result
1	1
1	2
1	3
1	4
1	1

主要就是那个

JOIN mysql.help_topic b ON b.help_topic_id < (
	length( a.`num` ) - length(
	REPLACE ( a.`num` , ',', '' ))+ 1);