根据最大日期返回具有多行的结果集

Question

我有一个类似于这样的子表：

[客户日期表]

| Customer ID | Some Date  | Balance |
+-------------+------------+---------+
|           1 | 2012-04-30 |   20.00 |
|           1 | 2012-03-31 |   50.00 |
|           2 | 2012-04-30 |    0.00 |
|           2 | 2012-03-31 |   10.00 | 
|           3 | 2012-03-31 |   60.00 |
|           3 | 2012-02-29 |   10.00 |

我希望能够获得这样的结果集 - 每个客户端的最新日期记录：

| Customer ID | Some Date  | Balance |
+-------------+------------+---------+
|           1 | 2012-04-30 |   20.00 | 
|           2 | 2012-04-30 |    0.00 |
|           3 | 2012-03-31 |   60.00 |

我知道我可以使用以下 SQL（SQL Server 语法）为每个单独的“客户 ID”执行此操作：

select top 1  [Some Date], [Customer ID], [Balance]
from [Cust Date Table]
where [Customer ID] = 2
order by [Some Date] desc


| Customer ID | Some Date  | Balance |
+-------------+------------+---------+
|           2 | 2012-04-30 |    0.00 |

但我不确定如何获得我想要的所有三张唱片。我不确定这是需要子查询还是其他什么的情况。

请注意，对于任何给定的 [客户 ID]，最大日期可能不同（在此示例中，客户 3 的最大日期为 2012-03-31，而其他记录的最大日期为 2012-04-30）。我试过了

select [Customer ID], MAX([Some Date]) AS [Latest Date], Balance 
from [Cust Date Table] 
group by [Customer ID], Balance; 

问题是这不仅为每个客户返回一行 - 它返回多行。

Answer 1

你只是想要：

SELECT
    [Customer ID],
    MAX([Some Date]) AS[Latest Date]
FROM[Cust Date TABLE]
GROUP BY
    [Customer ID];

好的 - 你已经修改了它。您现在要对行进行排序并选择最上面的一行：

WITH numbered AS (
    SELECT
        [Customer ID],
        [Some Date],
        [Balance],
        ROW_NUMBER() OVER (
            PARTITION BY
                [Customer ID]
            ORDER BY
                [Some Date] DESC
        ) AS rownum
    FROM[Cust Date TABLE]
)
SELECT
    [Customer ID],
    [Some Date],
    [Balance]
FROM numbered
WHERE
    rownum = 1;

Answer 2

我想你在追求这样的事情

select c.[customer ID], [some date], balance
from [cust date table] c
inner join 
    ( select [customer ID], MAX([some date]) as maxdate
    from [cust date table]
    group by [customer ID]) c2
on c2.[customer ID] = c.[customer ID]
and c2.maxdate = c.[some date]

对此有许多变体，即 CTE、表变量、#table，您可以使用这些变体来查看在您的情况下什么可以为您提供最佳性能。