UPDATE users as u2 SET u2.`password` = 
SELECT u.password as pwd FROM homestead.users as u LEFT JOIN homestead.teachers as t on u.id = t.user_id WHERE t.staff_no = '2003001415' LIMIT 1)
WHERE u2.`loginname` = 'admin' 
LIMIT 1

这样会报错，是mysql本身不支持操作本身又把本身作为数据源（一眼看上去特别有真理，但是看看语句逻辑，在别的数据库是先查出数据，再更新数据，不矛盾呀，也许是mysql查询的时候有点懒，没把查询的结果做一个隔离，而是映射什么的）

解决办法：（包一层就解决了）

UPDATE users as u2 SET u2.`password` = 
    (SELECT pwd from (
        SELECT u.password as pwd FROM homestead.users as u LEFT JOIN homestead.teachers as t on u.id = t.user_id WHERE t.staff_no = '2003001415' LIMIT 1) as uu) 
WHERE u2.`loginname` = 'admin' LIMIT 1