oracle，把一个表的某字段值根据关联条件赋值给另外一张表的字段

MERGE into STUDENTS_INFO S
using (
select si.ID_NUMBER,p.POLICYFLAG,p.PAY_TIME  
from STUDENTS_INFO si
left join orders o on si.ID_NUMBER = o.CID
LEFT join policies p ON p.ORDER_ID = o.ORDER_ID
where p.POLICYFLAG = '4'
and p.EFFECT_TIME <= SYSDATE
and p.END_TIME > sysdate) T
on (S.ID_NUMBER = T.ID_NUMBER)
when matched then
update set S.POLICYFLAG = T.POLICYFLAG,S.PAY_TIME = T.PAY_TIME;