Sql server TSQL求和基于双ID值的值

这是我的数据的一小部分简化片段

OrderID  QTY  ItemID  ActualQTY(does not exist in database)
1        2    1       
2        1    2
3        1    1
4        5    3

现在我需要一个查询，它将根据

ItemID的

填充

ActualQTY

。因此，将

itemid1=3

和

itemid2=1

的总数量相加，最后将

itemid3=5

应该是这样的

OrderID  QTY  ItemID  ActualQTY
1        2    1       3
2        1    2       1
3        1    1       3
4        5    3       5

问题是我是TSQL新手，我想不出一个好的方法来实现这一点

--编辑--

其他人帮我解决了这个问题，并给出了这个对我来说似乎是最有效的解决方案。但是，如果需要将它们应用于VisualStudio中的XSD文件，则此解决方案不起作用。因此，我将其转换为服务器上的表值函数

SELECT OrderID, QTY, ItemID, SUM(QTY) OVER(PARTITION BY ItemID) AS ActualQTY

因此，如果此解决方案不起作用，请求助于下面的答案，您可以这样做：

SELECT b.OrderID,
       b.QTY,
       b.ItemID,
       a.total_sum AS ActualQTY
FROM   (SELECT ItemID,
               Sum(QTY) AS total_sum
        FROM   tablename
        GROUP  BY ItemID) AS a
       INNER JOIN tablename b
               ON a.ItemID = b.ItemID 

SELECT
  t2.OrderID,
  t2.QTY,
  t2.ItemID,
  t1.ActualQTY
FROM
(
   SELECT 
     ItemId,
     SUM(QTY) AS ActualQTY
   FROM tablename AS t1
   GROUP BY ItemId
) AS t1
INNER JOIN tablename AS t2 ON t1.ItemId = t2.ItemID;

UPDATE t1
SET t1.ActualQTY = t2.ActualQTY
FROM tablename AS t1
INNER JOIN
(
   SELECT 
     ItemId,
     SUM(QTY) AS ActualQTY
   FROM tablename AS t1
   GROUP BY ItemId
) AS t2 ON t1.ItemID = t2.ItemID

---

但是，如果要更新列

ActualQTY

值，而不仅仅是选择，可以执行以下操作：

SELECT
  t2.OrderID,
  t2.QTY,
  t2.ItemID,
  t1.ActualQTY
FROM
(
   SELECT 
     ItemId,
     SUM(QTY) AS ActualQTY
   FROM tablename AS t1
   GROUP BY ItemId
) AS t1
INNER JOIN tablename AS t2 ON t1.ItemId = t2.ItemID;

UPDATE t1
SET t1.ActualQTY = t2.ActualQTY
FROM tablename AS t1
INNER JOIN
(
   SELECT 
     ItemId,
     SUM(QTY) AS ActualQTY
   FROM tablename AS t1
   GROUP BY ItemId
) AS t2 ON t1.ItemID = t2.ItemID