如何加入到第一行

我将使用一个具体但假设的例子。

每个订单通常只有一个行项目:

订单:

OrderGUID   OrderNumber
=========   ============
{FFB2...}   STL-7442-1      
{3EC6...}   MPT-9931-8A

LineItems:

LineItemGUID   Order ID Quantity   Description
============   ======== ========   =================================
{098FBE3...}   1        7          prefabulated amulite
{1609B09...}   2        32         spurving bearing

但偶尔会有两行项目的订单:

LineItemID   Order ID    Quantity   Description
==========   ========    ========   =================================
{A58A1...}   6,784,329   5          pentametric fan
{0E9BC...}   6,784,329   5          differential girdlespring

通常在向用户显示订单时:

SELECT Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM Orders
    INNER JOIN LineItems 
    ON Orders.OrderID = LineItems.OrderID

我想展示订单上的单品。但如果偶尔有包含两种(或更多)商品的订单，订单就会出现重复:

OrderNumber   Quantity   Description
===========   ========   ====================
STL-7442-1    7          prefabulated amulite
MPT-9931-8A   32         spurving bearing
KSG-0619-81   5          panametric fan
KSG-0619-81   5          differential girdlespring

我真正想要的是有SQL Server只是选择一个，因为它将足够好:

OrderNumber   Quantity   Description
===========   ========   ====================
STL-7442-1    7          prefabulated amulite
MPT-9931-8A   32         differential girdlespring
KSG-0619-81   5          panametric fan

如果我大胆一点，我可能会向用户显示一个省略号，以表明有不止一个:

OrderNumber   Quantity   Description
===========   ========   ====================
STL-7442-1    7          prefabulated amulite
MPT-9931-8A   32         differential girdlespring
KSG-0619-81   5          panametric fan, ...

所以问题是如何二者兼得

消除“重复”行仅连接其中一行，以避免重复

第一次尝试

我第一次天真的尝试是只加入“TOP 1”行项目:

SELECT Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM Orders
    INNER JOIN (
       SELECT TOP 1 LineItems.Quantity, LineItems.Description
       FROM LineItems
       WHERE LineItems.OrderID = Orders.OrderID) LineItems2
    ON 1=1

但这就给出了错误:

列或前缀“Orders”则不是与表名或别名匹配用于查询。

大概是因为内部选择没有看到外部表。

当前回答

我知道这个问题已经回答了一段时间了，但是在处理大型数据集时，嵌套查询的成本可能很高。下面是一种不同的解决方案，其中嵌套查询将只运行一次，而不是为返回的每一行运行。

SELECT 
  Orders.OrderNumber,
  LineItems.Quantity, 
  LineItems.Description
FROM 
  Orders
  INNER JOIN (
    SELECT
      Orders.OrderNumber,
      Max(LineItem.LineItemID) AS LineItemID
    FROM
      Orders INNER JOIN LineItems
      ON Orders.OrderNumber = LineItems.OrderNumber
    GROUP BY Orders.OrderNumber
  ) AS Items ON Orders.OrderNumber = Items.OrderNumber
  INNER JOIN LineItems 
  ON Items.LineItemID = LineItems.LineItemID

2012-04-06 21:25:16

其他回答

相关子查询是依赖于外部查询的子查询。它就像SQL中的for循环。子查询将为外层查询中的每一行运行一次:

select * from users join widgets on widgets.id = (
    select id from widgets
    where widgets.user_id = users.id
    order by created_at desc
    limit 1
)

2016-09-17 10:19:44

SELECT 
  Orders.OrderNumber,
  LineItems.Quantity, 
  LineItems.Description
FROM 
  Orders
  INNER JOIN (
    SELECT
      Orders.OrderNumber,
      Max(LineItem.LineItemID) AS LineItemID
    FROM
      Orders INNER JOIN LineItems
      ON Orders.OrderNumber = LineItems.OrderNumber
    GROUP BY Orders.OrderNumber
  ) AS Items ON Orders.OrderNumber = Items.OrderNumber
  INNER JOIN LineItems 
  ON Items.LineItemID = LineItems.LineItemID

2012-04-06 21:25:16

SELECT   Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM     Orders
JOIN     LineItems
ON       LineItems.LineItemGUID =
         (
         SELECT  TOP 1 LineItemGUID 
         FROM    LineItems
         WHERE   OrderID = Orders.OrderID
         )

在SQL Server 2005及以上版本中，你可以用CROSS APPLY替换INNER JOIN:

SELECT  Orders.OrderNumber, LineItems2.Quantity, LineItems2.Description
FROM    Orders
CROSS APPLY
        (
        SELECT  TOP 1 LineItems.Quantity, LineItems.Description
        FROM    LineItems
        WHERE   LineItems.OrderID = Orders.OrderID
        ) LineItems2

请注意，没有ORDER BY的TOP 1是不确定的:这个查询将为每个订单提供一个行项目，但没有定义将是哪个行项目。

查询的多次调用可以为相同的订单提供不同的行项，即使底层没有更改。

如果想要确定顺序，应该在最里面的查询中添加一个order BY子句。

例子sqlfiddle

2010-01-11 16:48:26

从SQL Server 2012和以后，我认为这将做的技巧:

SELECT DISTINCT
    o.OrderNumber ,
    FIRST_VALUE(li.Quantity) OVER ( PARTITION BY o.OrderNumber ORDER BY li.Description ) AS Quantity ,
    FIRST_VALUE(li.Description) OVER ( PARTITION BY o.OrderNumber ORDER BY li.Description ) AS Description
FROM    Orders AS o
    INNER JOIN LineItems AS li ON o.OrderID = li.OrderID

2018-11-28 11:55:08

试过交叉，效果不错，但耗时稍长。调整行列有最大和增加组保持速度和下降额外的记录。

下面是调整后的查询:

SELECT Orders.OrderNumber, max(LineItems.Quantity), max(LineItems.Description)
FROM Orders
    INNER JOIN LineItems 
    ON Orders.OrderID = LineItems.OrderID
Group by Orders.OrderNumber

2013-02-14 21:47:20

如何加入到第一行

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