谁帮我看看这个语句哪里不对？

[qq_37753824]

Create table #tabl
(
Id int,
Amount float,
PId int
)
insert into #tabl values(1,100,null)
insert into #tabl values(2,200,null)
insert into #tabl values(3,null,1)
insert into #tabl values(4,null,2)

update #tabl set Amount=a.Amount
from #tabl a
where a.Id=PId
and Amount is null

select * from #tabl
drop table #tabl
为什么我这个update语句没作用？

[Dear SQL(燊)]

Create table #tabl
(
Id int,
Amount float,
PId int
)
insert into #tabl values(1,100,null)
insert into #tabl values(2,200,null)
insert into #tabl values(3,null,1)
insert into #tabl values(4,null,2)

update #tabl set Amount=a.Amount
from #tabl a
where a.Id=PId
and Amount is null

update b set Amount=a.Amount
from #tabl a
,#tabl b
where a.Id=b.PId
and b.Amount is null

select * from #tabl
drop table #tabl

/*
Id          Amount                 PId
----------- ---------------------- -----------
1           100                    NULL
2           200                    NULL
3           100                    1
4           200                    2

(4 行受影响)
*/

[Dear SQL(燊)]

Create table #tabl
(
Id int,
Amount float,
PId int
)
insert into #tabl values(1,100,null)
insert into #tabl values(2,200,null)
insert into #tabl values(3,null,1)
insert into #tabl values(4,null,2)

update b set Amount=a.Amount
from #tabl a
,#tabl b
where a.Id=b.PId
and b.Amount is null

select * from #tabl
drop table #tabl

[qq_37753824]

引用 2 楼 Dear SQL 的回复:

Create table #tabl
(
Id int,
Amount float,
PId int
)
insert into #tabl values(1,100,null)
insert into #tabl values(2,200,null)
insert into #tabl values(3,null,1)
insert into #tabl values(4,null,2)

update b set Amount=a.Amount
from #tabl a
,#tabl b
where a.Id=b.PId
and b.Amount is null

select * from #tabl
drop table #tabl

谢谢哥们 原来是这样啊。。