将当前的Mysql表中最大id值，更新为其它字段的值

首先借用以下楼上的　

UPDATE comment_global 
SET comment_target_id = (select id from (
    SELECT max(id) as id FROM comment_global
) as c)
 

其次你是不是没有条件限制
comment_target_id 的值全部改成id的值吗？
还是说id是条件
comment_target_id 的值改成id 的值

UPDATE comment_global 
SET comment_target_id =(SELECT id FROM (
    SELECT MAX(id) AS id FROM comment_global
) AS c)
WHERE id=
(SELECT id FROM (
    SELECT MAX(id) AS id FROM comment_global
) AS c)