if OBJECT_id('tb') is not null Drop table tb;
create table tb(id int,[date] datetime)
insert into TB values(810,'2004-02-15')
insert into TB values(810,'2004-03-02')
insert into TB values(810,'2004-04-18')
insert into TB values(810,'2004-05-06')
insert into TB values(810,'2004-06-20')
insert into TB values(450,'2004-03-22')
insert into TB values(450,'2005-06-22')
insert into TB values(450,'2005-07-22')
insert into TB values(520,'2004-03-02')
insert into TB values(520,'2004-03-22');

--取date_fin='2004-04-25'
declare @date_fin datetime;
set @date_fin = '2004-04-25';
with T1(id, date) as
(
    select id, min(date) from tb
    where date >= @date_fin
    group by id
)
select * from T1
Union all
select a.id, max(a.date) 
from tb a
where a.date < @date_fin
    and (not exists(select 1 from T1 where a.id = T1.id))
group by a.id
/*
id          date
----------- -----------------------
450         2005-06-22 00:00:00.000
810         2004-05-06 00:00:00.000
520         2004-03-22 00:00:00.000
*/

------解决方案--------------------
SQL code
--取date_fin='2004-04-25'

select distinct B.id,B.[date] --大于日期最近的 ，也可以两种写法交换。（修改后）
from TB A
cross apply(select top 1 id,[date] from TB where A.id =id and [date]>'2004-04-25' order by id,[date] ) B

union all

select id,max([date]) as date --小于日期最近的
from TB A
where not exists(select 1 from TB where id = A.id and [date] >'2004-04-25' )
group by id

/*
450    2005-06-22 00:00:00.000
810    2004-05-06 00:00:00.000*/

------解决方案--------------------
SQL code
create table TB(id int,[date] datetime)
insert into TB values(810,'2004-02-15')
insert into TB values(810,'2004-03-02')
insert into TB values(810,'2004-04-18')
insert into TB values(810,'2004-05-06')
insert into TB values(810,'2004-06-20')
insert into TB values(450,'2005-06-22')

declare @date char(10)
declare @sql varchar(100)
set @date='2005-08-10'
 
 if  exists (select *  from tb where date >=''+@date+'')
  begin
     set @sql=' select id from tb where date=(select min(date) as date from tb where date >='''+@date+''')'
  end
 else
  begin
       set @sql='select id from tb where date=(select max(date) as date from tb where date <='''+@date+''')'
  end
print @sql
exec (@sql)