求助高手 触发器不起作用 哪的问题？

在Sold表上建立一个插入触发器，在添加一个订单时，减少Book表的相应货物的记录的库存量。 可是这个触发器没起作用啊 哪出问题了？

create trigger Update_SellBook
on Sold
after insert
as 
update Book set Amount=Amount-1
from Book,inserted
where Book.BID=inserted.BID

SQL code

create table sold(bid int,col varchar(10))
create table book(bid int,amount int)
insert into book select 3,20
go
create trigger Update_SellBook
on Sold
after insert
as  
update b set Amount=Amount-1
from Book b,inserted
where b.BID=inserted.BID
go
insert into sold select 3,'aa'
select * from book
go
drop table book,sold
/*
bid         amount
----------- -----------
3           19

(1 行受影响)
*/

好象没看出什么问题..

SQL code

create trigger Update_SellBook
on Sold
after insert
as 
update Book set Amount=a.Amount-1
from Book a,inserted b
where a.BID=b.BID