获取多行

Question

的SQL最大的datetime

此语句返回2行：

SELECT ts.UnitId,ts.TeststepId, MAX(ts.CreatedAt)as CreatedAt 
FROM Teststep ts 
INNER JOIN Unit u ON ts.UnitId = u.UnitId 
Where u.TemplateId = 2 
Group by TeststepId, ts.UnitId 

我怎样才能获得一个行最高CreatedAt日期？我应该区分unitId？

UPDATE

亚伦和flem aksed更多信息：

1 Template has N Units 
1 Unit has N Teststeps 

的一步步测试表的聚集主键是TeststepId和CreatedAt。

[TeststepId] [int] NOT NULL, 

[Name] [nvarchar](50) NOT NULL, 

[PreCondition] [nvarchar](500) NOT NULL, 

[TestInstruction] [nvarchar](500) NOT NULL, 

[ExpectedResult] [nvarchar](500) NOT NULL, 

[CreatedAt] [datetime] NOT NULL, 

[CreatedBy] [nvarchar](50) NOT NULL, 

[UnitId] [int] NOT NULL, 

与在一步步测试表中的最重要领域的一步步测试表

样本数据可能是：

的一步步测试表明历史化teststeps。从上面的图片可以看出TeststepId = 4存在2次。历史版本（58秒）和当前版本（59秒）。

我想只用59秒的时间才能在“当前”视图中显示teststepid 4。

更新2

我想只有teststepid 4 59秒 所有其他teststepId s with their current date. If there are only rows with different teststepid。就拿那些，因为它们是当前行。

我正要改变我的集成测试并找到解决方案。

Answer 1

SELECT TOP (1) ts.UnitId, ts.TeststepId, ts.CreatedAt 
    FROM dbo.Teststep AS ts 
    INNER JOIN Unit AS u 
    ON ts.UnitId = u.UnitId 
    WHERE u.TemplateId = 2 
    ORDER BY ts.CreatedAt DESC; 

如果你想每单元ID一排，和SQL Server 2005 +，则：

;WITH x AS 
(
    SELECT ts.UnitId, ts.TestStepId, ts.CreatedAt, 
    rn = ROW_NUMBER() OVER (PARTITION BY ts.UnitId ORDER BY ts.CreatedAt DESC) 
    FROM dbo.Teststep AS ts 
    INNER JOIN Unit AS u 
    ON ts.UnitId = u.UnitId 
    WHERE u.TemplateId = 2 
) 
SELECT UnitId, TestStepId, CreatedAt 
    FROM x 
    WHERE rn = 1; 

Answer 2

SELECT TOP 1 x.* FROM 
    (SELECT ts.UnitId,ts.TeststepId, MAX(ts.CreatedAt)as CreatedAt FROM Teststep ts 
     INNER JOIN Unit u ON ts.UnitId = u.UnitId 
     Where u.TemplateId = 2 
     Group by TeststepId, ts.UnitId 
     ORDER BY ts.CreatedAt DESC) as [x] 

Answer 3

另一种选择

试试这个：

WITH Data AS 
(
    SELECT ts.UnitId, ts.TeststepId, ROW_NUMBER() OVER(PARTITION BY UnitId ORDER BY ts.CreatedAt DESC) Position 
     FROM Teststep ts INNER JOIN Unit u ON ts.UnitId = u.UnitId 
    WHERE u.TemplateId = 2 
) 
SELECT * 
    FROM Data 
WHERE Position = 1 

Answer 4

另一方式：

with cte as (
SELECT ts.UnitId,ts.TeststepId, MAX(ts.CreatedAt)as CreatedAt 
FROM Teststep ts 
INNER JOIN Unit u ON ts.UnitId = u.UnitId 
Where u.TemplateId = 2 
Group by TeststepId, ts.UnitId) 
select ts.UnitId,ts.TeststepId, CreatedAt from cte where CreatedAt 
=(select MAX(CreatedAt) from cte)