How to duplicate a parent-child relationship with update?

Question

MySQL

The table has an id and parent_id field, this is a parent-child relationship, it is not filled in, more precisely, only id is filled. And there is a link code and parent_code, it is filled. Question - how do I duplicate the code-parent_code relationship for the id-parent_id fields

Answer 1

UPDATE `table` AS `t1`
  JOIN `table` AS `t2` ON `t2`.`code` = `t1`.`parent_code`
  SET `t1`.`parent_id` = `t2`.`id`