根据 DateTime 字段选择最新结果

Question

我有一个只有 4 个字段的简单表格。

http://sqlfiddle.com/#!3/06d7d/1

CREATE TABLE Assessment (
  id INTEGER IDENTITY(1,1) PRIMARY KEY,
  personId INTEGER NOT NULL,
  dateTaken DATETIME,
  outcomeLevel VARCHAR(2)
)

INSERT INTO Assessment (personId, dateTaken, outcomeLevel)
VALUES (1, '2014-04-01', 'L1')

INSERT INTO Assessment (personId, dateTaken, outcomeLevel)
VALUES (1, '2014-04-05', 'L2')

INSERT INTO Assessment (personId, dateTaken, outcomeLevel)
VALUES (2, '2014-04-03', 'E3')

INSERT INTO Assessment (personId, dateTaken, outcomeLevel)
VALUES (2, '2014-04-07', 'L1')

我正在尝试根据 dateTaken 为每个“personId”选择他们的最新评估结果。

所以我想要的以下数据的输出是。

[personId, outcomeLevel]
[1, L2]
[2, L1]

谢谢， 丹尼

Answer 1

这是使用公用表表达式的可能解决方案：

WITH cte AS (
SELECT
    ROW_NUMBER() OVER (PARTITION BY personId ORDER BY dateTaken DESC) AS rn
    , personId
    , outcomeLevel
FROM
    [dbo].[Assessment]
)
SELECT
    personId
    , outcomeLevel
FROM
    cte
WHERE
    rn = 1

关于 CTE

可以将公用表表达式 (CTE) 视为在单个 SELECT、INSERT、UPDATE、DELETE 或 CREATE VIEW 语句的执行范围内定义的临时结果集。 CTE 类似于派生表，因为它不存储为对象，并且仅在查询期间持续存在。与派生表不同，CTE 可以是自引用的，并且可以在同一查询中多次引用。 From MSDN: Using Common Table Expressions

Answer 2

SELECT asst.personId,
       asst.outcomeLevel 
FROM dbo.Assessment asst
WHERE  asst.dateTaken=(SELECT MAX(ast.dateTaken) 
                       FROM assessment ast
                       WHERE asst.personid=ast.personId)
ORDER BY asst.personId

结果会是这样的

personId 结果级别 1 L2 2 L1

Answer 3

如果没有 cte，这应该可以完美地工作：

SELECT [Table4].[personId], [Table4].[outcomeLevel]
FROM (
    SELECT [Table1].[personId]
    FROM [Assessment] AS [Table1]
    GROUP BY [Table1].[personId]
    ) AS [Table2]
CROSS APPLY (
    SELECT TOP (1) [Table3].[personId], [Table3].[outcomeLevel], [Table3].[dateTaken]
    FROM [Assessment] AS [Table3]
    WHERE [Table2].[personId] = [Table3].[personId]
    ORDER BY [Table3].[dateTaken] DESC
    ) AS [Table4]
ORDER BY [Table4].[dateTaken] DESC

Answer 4

;with Cte as (Select personId,outcomeLevel, C= ROW_NUMBER()
            over(PARTITION By personId Order By dateTaken desc)
            From #Assessment
            )

Select * from cte where C=1

示例here

Answer 5

试试这个：

SELECT a.personId, a.outcomeLevel
FROM Assessment a
INNER JOIN 
  (
    SELECT max(dateTaken) as datetaken1, personId 
    FROM Assessment
    GROUP BY personId ) b 
ON a.dateTaken = b.datetaken1

演示：http://sqlfiddle.com/#!3/06d7d/9

想法是首先导出一个包含每个人的最大日期的表格，然后将其与日期字段上的原始表格连接起来，这样您就可以获得这个最大日期的结果级别...

Answer 6

试试这个：

;with cte as
(select personId pid, max(dateTaken) maxdate
 from assessment
 group by personId)

select personId, outcomeLevel
from assessment a
inner join cte c on a.personId = c.pid
where c.maxdate = a.dateTaken
order by a.personId