The column prefix 't3 'does not...

Explain the following SQL statement
Update o10yssk set yggh = t4.em _ code
From o10yssk T1 inner join
(Select distinct t1.em _ No, max (t1. I _ date) as rq0
From code_no_tab T1 inner join o10yssk T2
On t1.em _ NO = t2.ygkh and t2.rq> = t1. I _ date and t2.rq = '2017-02-28 'group by t1.em _ No) T3
On t1.ygkh = t3.em _ No and t1.rq> = t3.rq0 inner join code_no_tab T4
On t1.ygkh = t4.em _ No and t3.rq0 = t4. I _ date and t1.rq = '2017-02-28'
As a result, the analysis has always been
Server: MSG 107, level 16, state 2, line 2
The column prefix 't3 'does not match with a table name or alias name used in the query.

Then change the update o10yssk set to update T1 set. Strange!