先进先出

 IF NOT OBJECT_ID('[tb]') IS NULL
    DROP TABLE [tb]
GO
CREATE TABLE [tb]([药品货号] NVARCHAR(10),[药品名称] NVARCHAR(10),[库存数量] INT,[批号] NVARCHAR(10))
INSERT [tb]
SELECT '000001',N'青霉素',2,'01' UNION ALL
SELECT '000001',N'青霉素',3,'02' UNION ALL
SELECT '000001',N'青霉素',5,'03' UNION ALL
SELECT '000001',N'青霉素',4,'04' UNION ALL
SELECT '000001',N'青霉素',6,'05'
GO
select [药品货号],[药品名称],[库存数量],[批号],identity(int,1,1) ID into # from [tb] 
select * from #
declare @Put int
set @Put=12 
select [药品货号],[药品名称],
case when (select sum([库存数量]) from # where [药品名称]=aa.[药品名称] and ID<=aa.ID)<@put then 0 else 
(select sum([库存数量]) from # where [药品名称]=aa.[药品名称] and ID<=aa.ID)-@put end as '[库存数量]'
from # aa 

药品货号       药品名称       [库存数量]
---------- ---------- -----------
000001     青霉素        0
000001     青霉素        0
000001     青霉素        0
000001     青霉素        2
000001     青霉素        8