仅显示最新行

Question

目标：
如果您检索列secondid中的数据的任何重复数据，那么您只能从最新日期检索一行。例如，在下面的数据中，我有两个不同的日期时间，我想从第二个列中的值6检索数据'2016-05-02 07：34：14.377'。

问题：
我的代码似乎不起作用，我错过了什么。

的信息：
有许多数据，您无法对代码中的值进行硬编码。

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

每Testing2Datetime Testing2Datetime的最大值是Testing2Datetime本身。您应该从group by子句中删除它，并仅按 secondid 检索最大值：

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

Answer 2

从bb.Testing2Datetime

中删除Group by列

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

甚至这个（Row_Number窗口函数）

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