mysql中You can't specify target table for update in FROM clause错误

     参考： https://www.jb51.net/article/60926.htm

mysql中You can't specify target table for update in FROM clause错误的意思是说，不能先select出同一表中的某些值，再update这个表(在同一语句中)。

      例如：将id=2且name不等于null的数据的name设置为'abc'，

首先要查出该数据再更新，如下：会报You can't specify target table for update in FROM clause错误！！！

UPDATE a SET name = 'abc' WHERE id = (select id from a where id = 2 and name is null);

改为：

update a set name = 'abc' where id = (select id from (select * from a where id = 2 and name is null) as b)