求一个SQL语句的编写

create table T3
(
PH varchar(20),
IM varchar(20),
DT datetime
)

insert into T3 values('A','IM1','2009-10-1 00:00:00')
insert into T3 values('A','IM2','2009-10-2 00:00:00')
insert into T3 values('A','IM3','2009-10-3 00:00:00')
insert into T3 values('B','IM2','2009-10-4 00:00:00')
insert into T3 values('C','IM2','2009-10-5 00:00:00')
insert into T3 values('C','IM4','2009-10-6 00:00:00')
insert into T3 values('D','IM4','2009-10-7 00:00:00')

现在想要如下结果：
'A','IM1','2009-10-1 00:00:00'
'A','IM3','2009-10-3 00:00:00'
'C','IM2','2009-10-5 00:00:00'
'D','IM4','2009-10-6 00:00:00'
（选择数据的要求：IM去重，同时选择每组IM值中对应DT值最大的行）

谢谢！

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not exists

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SQL code:

select * from T3 a where not exists(select 1 from T3 where a.IM=IM and a.DT<DT)

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SQL code:
select *
from tb t
where
not exists(select