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

我最喜欢使用not exists子句来运行这个查询。我相信这是运行这类查询的最有效的方法:

select o.OrderNumber,
       li.Quantity,
       li.Description
from Orders as o
inner join LineItems as li
on li.OrderID = o.OrderID
where not exists (
    select 1
    from LineItems as li_later
    where li_later.OrderID = o.OrderID
    and li_later.LineItemGUID > li.LineItemGUID
    )

但是我没有将此方法与这里建议的其他方法进行测试。

2017-05-09 18:12:59

其他回答

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

，另一种使用公共表表达式的方法:

with firstOnly as (
    select Orders.OrderNumber, LineItems.Quantity, LineItems.Description, ROW_NUMBER() over (partiton by Orders.OrderID order by Orders.OrderID) lp
    FROM Orders
        join LineItems on Orders.OrderID = LineItems.OrderID
) select *
  from firstOnly
  where lp = 1

或者，在最后，也许你想显示所有行连接?

逗号分隔的版本:

  select *
  from Orders o
    cross apply (
        select CAST((select l.Description + ','
        from LineItems l
        where l.OrderID = s.OrderID
        for xml path('')) as nvarchar(max)) l
    ) lines

2017-05-10 10:01:10

我最喜欢使用not exists子句来运行这个查询。我相信这是运行这类查询的最有效的方法:

select o.OrderNumber,
       li.Quantity,
       li.Description
from Orders as o
inner join LineItems as li
on li.OrderID = o.OrderID
where not exists (
    select 1
    from LineItems as li_later
    where li_later.OrderID = o.OrderID
    and li_later.LineItemGUID > li.LineItemGUID
    )

但是我没有将此方法与这里建议的其他方法进行测试。

2017-05-09 18:12:59

编辑:没关系，Quassnoi有更好的答案。

对于SQL2K，如下所示:

SELECT 
  Orders.OrderNumber
, LineItems.Quantity
, LineItems.Description
FROM (  
  SELECT 
    Orders.OrderID
  , Orders.OrderNumber
  , FirstLineItemID = (
      SELECT TOP 1 LineItemID
      FROM LineItems
      WHERE LineItems.OrderID = Orders.OrderID
      ORDER BY LineItemID -- or whatever else
      )
  FROM Orders
  ) Orders
JOIN LineItems 
  ON LineItems.OrderID = Orders.OrderID 
 AND LineItems.LineItemID = Orders.FirstLineItemID

2010-01-11 16:59:58

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

如何加入到第一行

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