使用变量更新时的奇怪更新行为

Question

这是对我的问题Reference a column and update it in the same statement 的跟进。我现在正在尝试使用局部变量并在同一个更新语句中对其进行更新。

declare @tmp table (ID int primary key, UNPAID money)

insert into @tmp select 1, 31.63
insert into @tmp select 2, 49.20
insert into @tmp select 3, 99.00
insert into @tmp select 4, 41.00

declare @paymentAmmount money
select @paymentAmmount = SUM(UNPAID) from @tmp
            
declare cur_oustandingAR cursor local static for select ID  from @tmp order by ID
open cur_oustandingAR

declare @currentID int

fetch next from cur_oustandingAR into @currentID
while (@@fetch_status = 0 and @paymentAmmount > 0)
begin
    begin
        update @tmp
            set UNPAID = case when @paymentAmmount > UNPAID then 0 else UNPAID - @paymentAmmount end,
                @paymentAmmount = case when @paymentAmmount > UNPAID then @paymentAmmount - UNPAID else 0 end
            where ID = @currentID
    end
    fetch next from cur_oustandingAR into @currentID
end

select * from @tmp
select @paymentAmmount as LeftoverPayment

您可以运行查询here，这是它给出的结果

身份证未付 ------------ --------- 1 0.00 2 0.00 3 58.00 4 41.00 剩余付款 --------------- 0

所有值都应为 0，最后的 @paymentAmmount 也应为 0。是什么问题导致无法正确应用这些值？

---

附：我知道如何fix it，只需将一个查询分解为以下 3 个查询，但我想将其作为一个查询进行，因此我不需要针对真实表进行尽可能多的查找

select @oldUnpaid = UNPAID from @tmp where ID = @currentID
update @tmp 
    set UNPAID = case when @paymentAmmount > UNPAID then 0 else UNPAID - @paymentAmmount end
    where ID = @currentID
select @paymentAmmount = case when @paymentAmmount > @oldUnpaid then @paymentAmmount - @oldUnpaid else 0 end

我只是想知道为什么我目前正在做的事情不起作用。

Answer 1

declare @tmp table (ID int primary key, UNPAID money)

insert into @tmp select 1, 31.63
insert into @tmp select 2, 49.20
insert into @tmp select 3, 99.00
insert into @tmp select 4, 41.00

declare @paymentAmmount money
declare @paymentAmmountbuf money
select @paymentAmmount = SUM(UNPAID) from @tmp
declare cur_oustandingAR cursor local static for select ID  from @tmp order by ID
open cur_oustandingAR

declare @currentID int

fetch next from cur_oustandingAR into @currentID
while (@@fetch_status = 0 and @paymentAmmount > 0)
begin
    begin
        select @paymentAmmountbuf=@paymentAmmount

        update @tmp
            set UNPAID = case when @paymentAmmountbuf > UNPAID then 0 else UNPAID - @paymentAmmountbuf end,
                @paymentAmmount = case when @paymentAmmount > UNPAID then @paymentAmmount - UNPAID else 0 end
            where ID = @currentID
    end
    fetch next from cur_oustandingAR into @currentID
end

select * from @tmp
select @paymentAmmount as LeftoverPayment