declare @t table([booksid] int,[datetime] datetime)
insert @t
select 1,'10:00:00' union all
select 1,'12:00:00' union all
select 2,'07:00:00' union all
select 2,'11:00:00'

select [booksid],
convert(varchar(8),[datetime],108) as [datetime] 
from @t t
where [datetime]=(select max([datetime]) from @t where [booksid]=t.[booksid])
order by 1
/*
booksid     datetime
----------- --------
1           12:00:00
2           11:00:00
*/

------解决方案--------------------
SQL code

declare @t table([booksid] int,[datetime] varchar(8))
insert @t
select 1,'10:00:00' union all
select 1,'12:00:00' union all
select 2,'07:00:00' union all
select 2,'11:00:00'

select * from @t a 
where not exists(select 1 from @t where [booksid]=a.[booksid] and [datetime]>a.[datetime])

/*
booksid     datetime
----------- --------
1           12:00:00
2           11:00:00
*/

------解决方案--------------------

参考：

SQL code

--> --> (Roy)生成測試數據
 
if not object_id('Tempdb..#T') is null
    drop table #T
Go
Create table #T([ID] int,[Name] nvarchar(1),[Memo] nvarchar(2))
Insert #T
select 1,N'A',N'A1' union all
select 2,N'A',N'A2' union all
select 3,N'A',N'A3' union all
select 4,N'B',N'B1' union all
select 5,N'B',N'B2'
Go


--I、Name相同ID最小的记录(推荐用1,2,3),方法3在SQl05时，效率高于1、2
方法1:
Select * from #T a where not exists(select 1 from #T where Name=a.Name and ID<a.ID)

方法2:
select a.* from #T a join (select min(ID)ID,Name from #T group by Name) b on a.Name=b.Name and a.ID=b.ID

方法3:
select * from #T a where ID=(select min(ID) from #T where Name=a.Name)

方法4:
select a.* from #T a join #T b on a.Name=b.Name and a.ID>=b.ID group by a.ID,a.Name,a.Memo having count(1)=1 

方法5:
select * from #T a group by ID,Name,Memo having ID=(select min(ID)from #T where Name=a.Name)

方法6:
select * from #T a where (select count(1) from #T where Name=a.Name and ID<a.ID)=0

方法7:
select * from #T a where ID=(select top 1 ID from #T where Name=a.name order by ID)

方法8:
select * from #T a where ID!>all(select ID from #T where Name=a.Name)

方法9(注:ID为唯一时可用):
select * from #T a where ID in(select min(ID) from #T group by Name)

--SQL2005:

方法10:
select ID,Name,Memo from (select *,min(ID)over(partition by Name) as MinID from #T a)T where ID=Min