Sql 如何选择每天最新的时间戳

我每小时都有一个报告，显示数据库的磁盘空间使用情况。 然后通过电子邮件发送

我想将其更改为显示上次快照日期的统计信息

数据如下

Day Date    Time    WeekNo  Data_Size_MB    Used_Data_MB

Friday  10-Mar-17   11:00   11  118784  92670.33

Friday  10-Mar-17   10:00   11  118784  92653.96

Friday  10-Mar-17   09:00   11  118784  92641.16

Friday  10-Mar-17   08:00   11  118784  92636.39

Friday  10-Mar-17   07:00   11  118784  92584.42

Thursday    09-Mar-17   23:00   11  118784  92586.25

Thursday    09-Mar-17   22:00   11  118784  92586.06

Thursday    09-Mar-17   21:00   11  118784  92585.85

下面的sql

select  Day = datename(dw,snapshot_Date),
        Date = convert(varchar(19), Snapshot_Date, 106),
        Time = substring(CONVERT(VARCHAR, snapshot_Date, 108),0,6),
        WeekNo = DATEPART(week, snapshot_Date),
        Data_Size_MB = Data_Size_MB,
        Used_Data_MB = Used_Data_MB,
        Data_Full_Pcnt = Data_Full_Pcnt,
        Log_Size_MB = Log_Size_MB,
        Free_Log_MB = Free_Log_MB,
        Log_Full_Pcnt = Log_Full_Pcnt,  
        snapshot_Date            
from Ideal_Risk.dbo.Sybase_Disk_Space
where database_name='IDEAL'
order by snapshot_date desc

但我希望它能回来

Day Date    Time    WeekNo  Data_Size_MB    Used_Data_MB

Friday  10-Mar-17   11:00   11  118784  92670.33

Thursday    09-Mar-17   23:00   11  118784  92586.25

ie每天的最后一张快照

---

注意：我已经切掉了数据中返回的一些列，解决方案是创建一个中间表，在其中每个日期添加一个行号。然后，每个日期只能选择第一行

select *, 
       ROW_NUMBER() OVER(PARTITION BY Date ORDER BY Date DESC) AS rownum 
into #some_temp
from Ideal_Risk.dbo.Sybase_Disk_Space
where database_name='IDEAL'

select  Day = datename(dw,snapshot_Date),
    Date = convert(varchar(19), Snapshot_Date, 106),
    Time = substring(CONVERT(VARCHAR, snapshot_Date, 108),0,6),
    WeekNo = DATEPART(week, snapshot_Date),
    Data_Size_MB = Data_Size_MB,
    Used_Data_MB = Used_Data_MB,
    Data_Full_Pcnt = Data_Full_Pcnt,
    Log_Size_MB = Log_Size_MB,
    Free_Log_MB = Free_Log_MB,
    Log_Full_Pcnt = Log_Full_Pcnt,  
    snapshot_Date            
from #some_temp
where database_name='IDEAL' and rownum = 1

---

当然，您也可以从（选择…）b执行嵌套的

 您可以像这样使用CTE

with mondayTable as(
select  Day = datename(dw,snapshot_Date),
        Date = convert(varchar(19), Snapshot_Date, 106),
        Time = substring(CONVERT(VARCHAR, snapshot_Date, 108),0,6),
        WeekNo = DATEPART(week, snapshot_Date),
        Data_Size_MB = Data_Size_MB,
        Used_Data_MB = Used_Data_MB,
        Data_Full_Pcnt = Data_Full_Pcnt,
        Log_Size_MB = Log_Size_MB,
        Free_Log_MB = Free_Log_MB,
        Log_Full_Pcnt = Log_Full_Pcnt,  
        snapshot_Date            
from Ideal_Risk.dbo.Sybase_Disk_Space
where database_name='IDEAL' and datename(dw,snapshot_Date) = 'Monday'
order by snapshot_date desc
)

tuesdayTable as(
select  Day = datename(dw,snapshot_Date),
        Date = convert(varchar(19), Snapshot_Date, 106),
        Time = substring(CONVERT(VARCHAR, snapshot_Date, 108),0,6),
        WeekNo = DATEPART(week, snapshot_Date),
        Data_Size_MB = Data_Size_MB,
        Used_Data_MB = Used_Data_MB,
        Data_Full_Pcnt = Data_Full_Pcnt,
        Log_Size_MB = Log_Size_MB,
        Free_Log_MB = Free_Log_MB,
        Log_Full_Pcnt = Log_Full_Pcnt,  
        snapshot_Date            
from Ideal_Risk.dbo.Sybase_Disk_Space
where database_name='IDEAL' and datename(dw,snapshot_Date) = 'Tuesday'
order by snapshot_date desc
)
SELECT  *
FROM    mondayTable
UNION ALL
SELECT  *
FROM    tuesdayTable

您应该能够使用
行编号（）
：

我们可以省略ROW_NUMBER（）函数的使用和每行的编号

在下面的查询中，我在CTE（
LatestDailySnapshot
）中查找按日期分组（
CAST（Snapshot\u Date AS Date）
）的每天的最大
Snapshot\u Date
），然后将结果连接到原始表以提取这些日期的记录

DECLARE @DatabaseName VARCHAR(50) = 'IDEAL';

WITH LatestDailySnapshot AS
(
    SELECT MAX(Snapshot_Date) AS Latest_Snapshot_Date
    FROM Ideal_Risk.dbo.Sybase_Disk_Space
    WHERE Database_Name = @DatabaseName
    GROUP BY CAST(Snapshot_Date AS Date)
)
SELECT *
FROM Ideal_Risk.dbo.Sybase_Disk_Space
WHERE Database_Name = @DatabaseName
JOIN LatestDailySnapshot ON Latest_Snapshot_Date = Snapshot_Date
ORDER BY snapshot_date

注1：如果在（
数据库名称、快照日期
）上有索引，则会更有效

注2：如果每个（
Database\u Name
，
Snapshot\u Date
）组合有多行，这种方法将无法正常工作。这意味着以下查询不应返回任何结果：

SELECT Database_Name, Snapshot_Date, COUNT(*) AS RowsPerCombination
FROM Ideal_Risk.dbo.Sybase_Disk_Space
GROUP BY Database_Name, Snapshot_Date
HAVING COUNT(*) > 1

SELECT Database_Name, Snapshot_Date, COUNT(*) AS RowsPerCombination
FROM Ideal_Risk.dbo.Sybase_Disk_Space
GROUP BY Database_Name, Snapshot_Date
HAVING COUNT(*) > 1