How to convert one row into multiple rows in mysql

Mysql method to turn one line into multiple lines: 1. To turn one line into multiple lines, the code is [substring_index(substring_index( a.rn,',',b.help_topic_id 1 ),',', - 1)]; 2. Convert multiple columns into multiple rows.

Mysql method to turn one row into multiple rows:

Put

Convert to

SELECT
    substring_index(substring_index( a.rn,',',b.help_topic_id + 1    ),',' ,- 1) AS rn
FROM
    (select '1,2,3,4' as rn) a  
JOIN mysql.help_topic b ON b.help_topic_id <
(length(a.rn) - length( replace(a.rn, &#39;,&#39;, &#39;&#39;)  ) + 1)

Convert Picture A to Picture B

Picture a

图b

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, &#39;,&#39;, &#39;&#39;)  ) + 1)

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