僅顯示最新的行

Question

目標：
如果您檢索任何重複的數據，即secondid列中的數據，則只能從最新日期檢索一行。例如，在下面的數據中，我有兩個不同的日期時間，我想從列secondid中的值6中檢索數據'2016-05-02 07：34：14.377'。

問題：
我的代碼似乎沒有工作，我錯過了什麼。

信息：
有許多數據，你不能硬編碼代碼中的值。

CREATE TABLE [dbo].[testing2](
    [id] [int] NOT NULL, 
    [secondid] [int] NULL, 
    [value] [varchar](30) NULL, 
    [category] [int] NULL, 
    [test_id] [int] NULL, 
    [id_type] [int] NOT NULL, 
    [Testing2Datetime] [datetime] not NULL, 
CONSTRAINT [PK_testing2] PRIMARY KEY CLUSTERED 
(
    [id] ASC 
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] 
) ON [PRIMARY] 


INSERT INTO [test].[dbo].[testing2] 
VALUES (3, 3, 'a' ,2 ,11 ,1, '2016-05-01 07:34:14.377'); 

INSERT INTO [test].[dbo].[testing2] 
VALUES (4, 4, 'a' ,2 ,11 ,1, '2016-05-01 07:34:14.377'); 

INSERT INTO [test].[dbo].[testing2] 
VALUES (5, 5, 'a' ,2 ,11 ,0, '2016-05-01 07:34:14.377'); 

INSERT INTO [test].[dbo].[testing2] 
VALUES (6, 6, 'a' ,2 ,11 ,2, '2016-05-01 07:34:14.377'); 

INSERT INTO [test].[dbo].[testing2] 
VALUES (7, 6, 'a' ,2 ,11 ,2, '2016-05-02 07:34:14.377'); 



select 
    bb.secondid, 
    max(bb.Testing2Datetime) 
from [dbo].[testing2] bb 
group by 
    bb.secondid, 
     bb.Testing2Datetime 

Answer 1

的Testing2Datetime最大值每柱本身就是Testing2Datetime。你應該從group by條款中刪除，而且只有secondid檢索最大值：

select 
    bb.secondid, 
    max(bb.Testing2Datetime) 
from [dbo].[testing2] bb 
group by 
    bb.secondid -- Here! 

Answer 2

取下Group by

select 
    bb.secondid, 
    max(bb.Testing2Datetime) as [Max_Testing2Datetime] 
from [dbo].[testing2] bb 
group by 
    bb.secondid 

或者甚至這個（Row_Number窗函數）

select * 
from 
(
select 
    bb.secondid, 
    bb.Testing2Datetime, 
    Row_number()over(partition by bb.secondid order by bb.Testing2Datetime desc) as RN 
from [dbo].[testing2] bb 
) A 
Where RN = 1