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create table test(t_a int,t_b int,t_c datetime)

insert into test(t_a,t_b)values (1,1,'2000-01-01')
insert into test(t_a,t_b)values (1,2,'2000-02-01')
insert into test(t_a,t_b)values (1,3,'2000-03-01')
insert into test(t_a,t_b)values (2,1,'2000-04-01')
insert into test(t_a,t_b)values (2,1,'2000-05-01')
insert into test(t_a,t_b)values (2,2,'2000-06-01')

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SQL code:
create table test(t_a int,t_b int,t_c datetime)

insert into test values (1,1,'2000-01-01')
insert into test values (1,2,'2000-02-01')
insert into test values (1,3,'2000-03-01')
insert into test values (2,1,'2000-04-01')
insert into test values (2,1,'2000-05-01')
insert into test values (2,2,'2000-06-01')

--·¨1
select * from test t where not exists(select 1 from test where t_a=t.t_a and t_c>t.t_c)
--·¨2
select * from test t where t_c =(select MAX(t_c) from test where t_a=t.t_a)
--·¨3
select * from test t where t_c =(select top 1 t_c from test where t_a=t.t_a order by t_c desc)

/*
t_a t_b t_c
----------- ----------- -----------------------
1 3 2000-03-01 00:00:00.000
2 2