oracle 无法修改与非键值保存表对应的列

update (select a.asset_id,a.qzf_qcyz,a.qzf_yz,a.qzf_ljzj,a.zfj_jz,y.zh from am_qzf a left join
(select asset_id,zh from am_qzf_ydzh where GJAHR=(select to_char(sysdate,'YYYY') from dual) and PERAF=(select to_char(sysdate,'MM') from dual)) y
on a.asset_id=y.asset_id) set qzf_ljzj=qzf_ljzj-zh,qzf_yz=qzf_yz+zh这个sql语句执行的时候总是提示标题错误？为什么？am_qzf 的asset_id 为该表的主键。