--> 测试数据：#PurchaseOrderMaster
if object_id('tempdb.dbo.#PurchaseOrderMaster') is not null drop table #PurchaseOrderMaster
create table #PurchaseOrderMaster(PurchaseOrderID int, PackageAmount numeric(10,2), InvoiceAmount numeric(10,2))
insert into #PurchaseOrderMaster
select 1, 300.00, 100.00 union all
select 2, 700.00, 400.00 union all
select 3, 500.00, 300.00
--> 测试数据：#PurchaseOrderItem
if object_id('tempdb.dbo.#PurchaseOrderItem') is not null drop table #PurchaseOrderItem
create table #PurchaseOrderItem(PurchaseOrderItemID int, PurchaseOrderID int, PackageSubTotalAmount numeric(10,2), ProRataPackageSubTotalAmount numeric(10,2))
insert into #PurchaseOrderItem
select 1, 1, 100.00, 0.00 union all
select 2, 1, 100.00, 0.00 union all
select 3, 1, 100.00, 0.00 union all
select 4, 2, 500.00, 0.00 union all
select 5, 2, 100.00, 0.00 union all
select 6, 2, 100.00, 0.00 union all
select 7, 3, 100.00, 0.00 union all
select 8, 3, 200.00, 0.00 union all
select 9, 3, 200.00, 0.00


;with cte(item,po,ia,maxid,v) as
(
    select a.PurchaseOrderItemID, a.PurchaseOrderID, b.InvoiceAmount, max(a.PurchaseOrderItemID)over(partition by a.PurchaseOrderID), convert(dec(10,2), PackageSubTotalAmount*InvoiceAmount/PackageAmount) from #PurchaseOrderItem a join #PurchaseOrderMaster b on a.PurchaseOrderID=b.PurchaseOrderID
), v as
(
    select item, v = case maxid when item then ia-(select sum(v) from cte where po=t.po and item<t.item) else v end from cte t
)
update #PurchaseOrderItem set ProRataPackageSubTotalAmount = v from #PurchaseOrderItem, v where PurchaseOrderItemID=item



select * from #PurchaseOrderItem
/*
PurchaseOrderItemID PurchaseOrderID PackageSubTotalAmount                   ProRataPackageSubTotalAmount
------------------- --------------- --------------------------------------- ---------------------------------------
1                   1               100.00                                  33.33
2                   1               100.00                                  33.33
3                   1