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，需要用到left join关联其他表，

但是不能直接关联，否则报错：错误如下：

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='' and g2.mergeid=g.mergeid
)
where g.ismerge='' and exists
(
    select 1
    from imim_gireqbillitems gi2
    left join imim_gireqbills g2 on g2.guid=gi2.guid
    where g2.ismerge='' 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=''
) 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='' 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='' and g2.mergeid=T.mergeid
)