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
)