如何加入子查询的第一行？

Question

我有一个发票表和一个键相关的相关数据的子表。特别是，对于每张发票，我只对子表中的第一个相关行感兴趣。鉴于我希望每个发票键都有一个相关行 - 我该如何完成？

Select i.[Invoice Number],
       c.[Carrier Name]
From Invoice i
    Left Join Carriers c on i.[InvoiceKey] = c.[InvoiceKey]
Where -- what?

我猜从语义上讲，我正在寻找类似于 Top 1 c.CarrierName Group by InvoiceKey 概念的东西（或者如果在 T-SQL 中可能的话，那将是什么概念。）

我考虑过对子查询进行左连接，但这似乎效率不高。有没有人有任何 T-SQL 技巧来有效地实现这一目标？

编辑：对不起，伙计们，我忘了提到这是 SQL Server 2000，所以虽然我要为当前可行的 SQL Server 2005/2008 响应投赞成票，但我不能恐怕不接受他们。

Answer 1

假设Carriers 有一个名为PRIMARY KEY 的id：

SELECT  i.[Invoice Number],
        c.[Carrier Name]
FROM    Invoice i
JOIN    Carriers c
ON      c.id = 
        (
        SELECT  TOP 1 ID
        FROM    Carriers ci
        WHERE   ci.InvoiceKey = i.InvoiceKey
        ORDER BY
                id -- or whatever
        )

Answer 2

这对我有用：

select ir.[Invoice Number], c.[Carrier Name]
from 
    (select ROW_NUMBER() over (order by i.[Invoice Number] asc) AS RowNumber, i.[Invoice Number], i.InvoiceKey
    from Invoice i) AS ir
left join Carriers c
on ir.InvoiceKey = c.InvoiceKey
where RowNumber = 1
union all
select ir.[Invoice Number], NULL as [Carrier Name]
from 
    (select ROW_NUMBER() over (order by i.[Invoice Number] asc) AS RowNumber, i.[Invoice Number]
    from Invoice i) AS ir
where RowNumber > 1

或

select TOP 1 i.[Invoice Number], c.[Carrier Name]
from Invoice i
left join Carriers c
on i.InvoiceKey = c.InvoiceKey
union all
select ir.[Invoice Number], NULL as [Carrier Name]
from 
    (select ROW_NUMBER() over (order by i.[Invoice Number] asc) AS RowNumber, i.[Invoice Number]
    from Invoice i) AS ir
where RowNumber > 1

Answer 3

我就是这样做的，使用的语法与你的稍有不同（MySQL 风格），但我想你也可以将它应用到你的解决方案中：

SELECT i.invoiceNumber, c.carrierName
FROM Invoice as i
LEFT JOIN Carriers as c ON (c.id = (SELECT id FROM Carriers WHERE invoiceKey = i.invoiceKey ORDER BY id LIMIT 1))

这将从 Invoice 中获取所有记录，并将其与 Carriers 中的一个（或零个）记录连接起来，特别是具有相同 invoiceKey 且只有第一个的记录。

只要您在 Carriers.invoiceKey 上有一个索引，这个查询的性能应该是可以接受的。

塞巴斯蒂安

Answer 4

;with cteRowNumber as (
    select c.InvoiceKey, c.[Carrier Name], ROW_NUMBER() over (partition by c.InvoiceKey order by c.[Carrier Name]) as RowNum
        from Carriers c
)
select i.[Invoice Number],
       rn.[Carrier Name]
    from Invoice i
        left join cteRowNumber rn
            on i.InvoiceKey = rn.InvoiceKey
                and rn.RowNum = 1

Answer 5

在这种情况下，我经常使用一种设备，我在此将其应用于您的示例并在下面进行描述：

SELECT
  i.[Invoice Number],
  c.[Carrier Name]
FROM Invoice i
  INNER JOIN Carriers c ON i.InvoiceKey = c.InvoiceKey
  INNER JOIN (
    SELECT MIN(ID) AS ID
    FROM Carriers
    GROUP BY InvoiceKey
  ) c_top ON c.ID = c_top.ID

我认为，这大致就是 Quassnoi 发布的内容，只是我尽量避免使用这样的 SELECT TOP。

Invoice 与Carriers 基于它们的链接表达式（在本例中为InvoiceKey）连接。现在，Carriers 可以有多个相同的InvoiceKey 行，所以我们需要限制输出。这是使用派生表完成的。

派生表根据用于链接两个表 (InvoiceKey) 的相同表达式对 Carrier 中的行进行分组。

还有另一种方法：您可以使用IN (subquery) 而不是加入派生表，效果相同。也就是说，完整的查询将如下所示：

SELECT
  i.[Invoice Number],
  c.[Carrier Name]
FROM Invoice i
  INNER JOIN Carriers c ON i.InvoiceKey = c.InvoiceKey
    AND c.ID IN (SELECT MIN(ID) FROM Carriers GROUP BY InvoiceKey)

Answer 6

group by carriername having max(invoicenumber)

获取每张发票的第一个承运人：

group by invoicenumber having max(carriername)
-- substitute the column you want to order by for carrier name to change which is 'first'

Answer 7

您也可以使用OUTER APPLY。 请注意对未知字段名称使用尖括号：

Select i.[Invoice Number], c.[Carrier Name], x.<Carrier_field1>
From Invoice i
OUTER APPLY 
(
    SELECT TOP 1
    FROM Carriers c 
    WHERE c.[InvoiceKey] = i.[InvoiceKey]
    ORDER BY <order _clause>
) x