CREATE TABLE #TEMP(D VARCHAR(20),QTYI INT,ID INT DEFAULT 0)
insert #TEMP(D,QTYI)
select 'A',1 union all
select 'A',4 union all
select 'A',3 union all
select 'A',3 union all
select 'A',5 union all
select 'A',2 union all
select 'A',3 UNION ALL
SELECT 'A',5 


DECLARE @INT INT =20
UPDATE #TEMP 
SET ID=case when QTYI-@INT>0 then qtyi else 0 end ,
   @int=case when QTYI-@INT >0 then 0 else @INT-qtyi end
   where @INT>0

select * from #TEMP
DROP TABLE #TEMP

CREATE TABLE #TEMP(D VARCHAR(20),QTYI INT,ID INT DEFAULT 0)
insert #TEMP(D,QTYI)
select 'A',1 union all
select 'A',4 union all
select 'A',3 union all
select 'A',3 union all
select 'A',5 union all
select 'A',2 union all
select 'A',3 UNION ALL
SELECT 'A',5 


DECLARE @INT INT ,@qty int,@n int
select @Int=20,@qty=0,@n=0
UPDATE #TEMP 
SET  
ID=case when @int>0 then case when @INT>=QTYI then qtyi else @INT end  else 0 end,
@int=case when @INT>=@qty  then @INT-@qty else 0 end,
@qty=qtyi
  


select * from #TEMP
--DROP TABLE #TEMP
/*D                    QTYI        ID          
-------------------- ----------- ----------- 
A                    1           1
A                    4           4
A                    3           3
A                    3           3
A                    5           5
A                    2           2
A                    3           2
A                    5           0

（所影响的行数为 8 行）

------解决方案--------------------
在定义一个变量

SQL code
CREATE TABLE #TEMP(D VARCHAR(20),QTYI INT,ID INT DEFAULT 0)
insert #TEMP(D,QTYI)
select 'A',1 union all
select 'A',4 union all
select 'A',3 union all
select 'A',3 union all
select 'A',5 union all
select 'A',2 union all
select 'A',3 UNION ALL
SELECT 'A',5 


DECLARE @INT INT =20,@a int
UPDATE #TEMP 
SET @a=@INT, @INT=@INT-QTYI, ID=case when @INT>0 then QTYI when @a>0 and @INT<0 then qtyi+@INT else 0 end 

select * from #TEMP
DROP TABLE #TEMP