获取多行的sql max datetime

Question

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

如何获得日期最长CreatedAt的一行？我应该区分unitId吗？

更新

Aaron和flem aksed获取更多信息：

1 Template has N Units
1 Unit has N Teststeps

Teststep表的聚簇主键是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,

Teststep 表

Teststep表中包含最重要字段的示例数据可能是：

测试步骤显示历史测试步骤。从上图中可以看出TeststepId = 4存在2次。历史版本（58秒）和当前版本（59秒）。

我想在59秒内只获得teststepid 4以在“当前”视图中显示。

更新2

我想只用59秒AND得到teststepid 4 所有其他teststepId s with their current date. If there are only rows with different teststepid取这些因为它们是当前行。

我即将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;   

如果你想要每个UnitId和SQL Server 2005+一行，那么：

;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

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 3

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 4

另一种方式：

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)