SQL Select 每组最新记录

Question

我正在尝试获取表中每个用户的最新记录：

SELECT *  
FROM Orders 
WHERE State = Active 
GROUP BY UserId
ORDER BY Orders.DateTimePlanned DESC`

但这导致我在每个用户的最旧记录中，我怎样才能获得最新的记录？ 将DESC更改为ASC不起作用！

请告诉我！

Answer 1

您的代码不是有效的标准 SQL。据推测，您正在运行 MySQL 并禁用 sql 模式ONLY_FULL_GROUP_BY 。

您需要过滤数据集而不是聚合它。 一个选项使用子查询：

select *  
from orders o
where state = 'Active' and datetimeplanned = (
    select max(o1.datetimeplanned)
    from orders o1
    where o1.userid = o.userid and o1.state = 'Active'
)

您还可以使用 window 函数（仅在 MySQL 8.0 中可用）：

select *  
from (
    select o.*, rank() over(partition by userid order by datetimeplanned desc) rn
    from orders o
    where state = 'Active'
) o
where rn = 1

Answer 2

我会使用子查询

看看这个脚本，它使用子查询并获取最后一行并使用前一行的值减少它以预测数据库的数据增长

声明@backupType char(1), @DatabaseName sysname

set @DatabaseName   = db_name() --> Name of current database, null for all databaseson server
set @backupType     ='D'        /* valid options are:
                                D = Database 
                                I = Database Differential 
                                L = Log 
                                F = File or Filegroup 
                                G = File Differential 
                                P = Partial 
                                Q = Partial Differential
                                */



select  backup_start_date
      , backup_finish_date
      , DurationSec
      , database_name,backup_size
      , PreviouseBackupSize
      , backup_size-PreviouseBackupSize as growth
      ,KbSec= format(KbSec,'N2')
FROM (
select backup_start_date 
     , backup_finish_date
     , datediff(second,backup_start_date,b.backup_finish_date) as DurationSec 
     , b.database_name
     , b.backup_size/1024./1024. as backup_size

     ,case when datediff(second,backup_start_date,b.backup_finish_date) >0 
        then ( b.backup_size/1024.)/datediff(second,backup_start_date,b.backup_finish_date)
        else 0 end as KbSec
--   , b.compressed_backup_size
     , (      
        select top (1) p.backup_size/1024./1024. 
          from msdb.dbo.backupset p 
         where p.database_name = b.database_name
           and p.database_backup_lsn< b.database_backup_lsn
            and type=@backupType
         order by p.database_backup_lsn desc
        ) as PreviouseBackupSize
from msdb.dbo.backupset as b
where @DatabaseName IS NULL OR database_name =@DatabaseName
and type=@backupType
)as A
order by backup_start_date desc