SQL Server:如何加入第一行

时间:2010-01-11 16:44:37

标签: sql sql-server tsql sql-server-2000

我将使用一个具体但假设的例子。

每个订单通常只有一个订单项

订单

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

了LineItem:

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

但偶尔会有一个包含两个订单项的订单:

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

我想在订单上显示单个项目。但是,如果此订单偶尔包含两个(或更多)商品,则出现 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

我真正想要的是让SQL Server 选择一个,因为它足够好

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

如果我喜欢冒险,我可能会向用户显示一个省略号,表示不止一个:

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

  • 消除“重复”行
  • 仅加入其中一行,以避免重复

首次尝试

我的第一个天真的尝试是只加入“ TOP 1 ”订单项:

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

但是这给出了错误:

  

列或前缀“订单”不是   匹配表名或别名
  在查询中使用。

大概是因为内部选择没有看到外表。

13 个答案:

答案 0 :(得分:1063)

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

SQL Server 2005及更高版本中,您只需将INNER JOIN替换为CROSS APPLY

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TOP 1的{​​{1}}不具有确定性:此查询您将获得每个订单一个订单项,但未定义哪个订单项。

对于同一订单,多次调用查询可以为您提供不同的订单项,即使底层证券没有更改。

如果您想要确定性顺序,则应在最内层查询中添加ORDER BY子句。

答案 1 :(得分:99)

我知道这个问题已经回答了一段时间,但是在处理大型数据集时,嵌套查询的代价很高。这是一个不同的解决方案,其中嵌套查询只运行一次,而不是返回每行。

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

答案 2 :(得分:26)

你可以这样做:

SELECT 
  Orders.OrderNumber, 
  LineItems.Quantity, 
  LineItems.Description
FROM 
  Orders INNER JOIN LineItems 
  ON Orders.OrderID = LineItems.OrderID
WHERE
  LineItems.LineItemID = (
    SELECT MIN(LineItemID) 
    FROM   LineItems
    WHERE  OrderID = Orders.OrderID
  )

这需要LineItems.LineItemID上的索引(或主键)和LineItems.OrderID上的索引,否则会很慢。

答案 3 :(得分:17)

@Quassnoi答案很好,在某些情况下(特别是如果外表很大),使用窗口函数可能会有更高效的查询,如下所示:

SELECT  Orders.OrderNumber, LineItems2.Quantity, LineItems2.Description
FROM    Orders
LEFT JOIN 
        (
        SELECT  LineItems.Quantity, LineItems.Description, OrderId, ROW_NUMBER()
                OVER (PARTITION BY OrderId ORDER BY (SELECT NULL)) AS RowNum
        FROM    LineItems

        ) LineItems2 ON LineItems2.OrderId = Orders.OrderID And RowNum = 1

有时您只需need to test哪个查询可以提供更好的效果。

答案 4 :(得分:11)

,另一种使用公用表表达式的方法:

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,最后可能你想显示所有加入的行?

以逗号分隔的版本:

  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

答案 5 :(得分:7)

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。对于外部查询中的每一行,子查询将运行一次:

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
)

答案 6 :(得分:6)

从SQL Server 2012起,我认为这可以解决问题:

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

答案 7 :(得分:5)

编辑:没关系,Quassnoi有更好的答案。

对于SQL2K,类似这样:

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

答案 8 :(得分:3)

我最喜欢的运行此查询的方法是使用not exists子句。我相信这是运行此类查询的最有效方式:

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
    )

但我没有针对此处建议的其他方法测试此方法。

答案 9 :(得分:2)

尝试过十字架,效果很好,但需要稍微长一点。调整后的行列具有最大值和添加组,这样可以保持速度并降低额外记录。

以下是经过调整的查询:

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

答案 10 :(得分:2)

我使用LEFT JOIN和GROUP BY Orders.OrderNumber解决了类似的问题。有没有理由不这样做?

SELECT Orders.OrderNumber, LineItems.Quantity, LineItems.Description
FROM Orders
    LEFT JOIN LineItems 
    ON Orders.OrderID = LineItems.OrderID
GROUP BY Orders.OrderNumber

我会在你自己的问题中回答你的回答问题:

Orders             LineItems
+-------------+    +---------+----------+---------------+
| OrderNumber |    | OrderID | Quantity | Description   |
+-------------+    +---------+----------+---------------+
| 22586       |    | 22586   | 17       | Trunion       |
+-------------+    | 22586   | 3        | Girdle Spring |
                   +---------+----------+---------------+

在OrderNumber上将两者连接起来给出:

OrderNumber  Quantity  Description
-----------  --------  -------------
22586        17        Trunion
22586        3         Girdle Spring

2 row(s) affected

我们希望它只返回一行:

OrderNumber  Quantity  Description
-----------  --------  -------------
22586        17        Trunion

1 row(s) affected

这就是我使用GROUP BY Orders.OrderNumber的原因,每个OrderNumber只返回一行。

答案 11 :(得分:1)

尝试

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

答案 12 :(得分:1)

CROSS APPLY 来救援:

SELECT Orders.OrderNumber, topline.Quantity, topline.Description
FROM Orders
cross apply
(
    select top 1 Description,Quantity
    from LineItems 
    where Orders.OrderID = LineItems.OrderID
)topline

您也可以添加您选择的 order by