oracle update left join查询

对于有的更新语句，要更新的表可能条件不够，须要用到left join关联其余表，

可是不能直接关联，不然报错：错误以下：

update  imim_gireqbillitems gi 
left join imim_gireqbills g on g.guid=gi.guid
set gi.quantity=
(
    select sum(gi2.quantity)
    from imim_gireqbillitems gi2
    left join imim_gireqbills g2 on g2.guid=gi2.guid
    where g2.ismerge='0' and g2.mergeid=g.mergeid
)
where g.ismerge='1' and exists
(
    select 1
    from imim_gireqbillitems gi2
    left join imim_gireqbills g2 on g2.guid=gi2.guid
    where g2.ismerge='0' and g2.mergeid=g.mergeid
)

正确实例：

update 
(
  select * from imim_gireqbillitems gi 
  left join imim_gireqbills g on g.guid=gi.guid
  where g.ismerge='1'
) T
set T.quantity=
(
    select sum(gi2.quantity)
    from imim_gireqbillitems gi2
    left join imim_gireqbills g2 on g2.guid=gi2.guid
    where g2.ismerge='0' and g2.mergeid=T.mergeid
)
where exists
(
    select 1
    from imim_gireqbillitems gi2
    left join imim_gireqbills g2 on g2.guid=gi2.guid
    where g2.ismerge='0' and g2.mergeid=T.mergeid
)