项目中有一个表是树形结构，并且准备把某一个节点的属性复制到下面的每个子节点。写好了 update 脚本，执行却报错了。

1093 - Table 'a' is specified twice, both as a target for 'UPDATE' and as a separate source for data

脚本大概长这样

update t_menu a set path = (select path from t_menu where id = a.parent_id )

在 stackoverflow 找到了答案，

Table is specified twice, both as a target for 'UPDATE' and as a separate source for data in mysql (opens new window)

简单点说就是再用一次子查询。将 SQL 改造成如下

update t_menu a set path = (select b.path from (select path from t_menu ) b where b.id = a.parent_id )