SELECT a.A,b.B FROM t1 AS a
    CROSS APPLY(SELECT TOP 1 B FROM t1 AS x WHERE x.A=a.A) AS b
GROUP BY a.A,b.B
ORDER BY a.A

------解决方案--------------------
;with cte as (
select rn=row_number() over(partition by a order by getdate()),* from t
)
select * from t where rn=1;


------解决方案--------------------
select   *   from   tb   a   where   b   in(select   top   1   b   from   tb   where   id=a.id)

------解决方案--------------------
SQL code
declare @test table(A int, B varchar(4))
insert into @test
select 1, 'x1' union
select 1, 'x2' union
select 2, 'x3' union
select 2, 'x4' union
select 2, 'xxx5' union
select 3, 'xxx' union
select 3, 'xxxx'
select A,B from
(
    select row_number() over(partition by A order by A) rn,* from @test
) t
where t.rn=1

/*
A           B
----------- ----
1           x1
2           x3
3           xxx
*/

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

-->测试数据
IF OBJECT_ID('tab') IS NOT NULL
DROP TABLE tab
GO
CREATE TABLE tab(A INT,B VARCHAR(10))
INSERT INTO tab

SELECT '1','x1' UNION ALL
SELECT '1','x2' UNION ALL
SELECT '2','x3' UNION ALL
SELECT '2','x4' UNION ALL
SELECT '2','xxx5' UNION ALL
SELECT '3','xxx' UNION ALL
SELECT '3','xxxx' 
GO
--select  * from tab
-->查询
;
with 
cte as (
select rn=row_number() over(partition by A order by B),* from tab
)
select * from cte where rn=1

/*
   A    B
   1    x1
   2    x3
   3    xxx
*/

------解决方案--------------------
SQL code
SELECT * FROM test a 
WHERE EXISTS (SELECT 1 FROM 
(SELECT A,NTILE(1) OVER(ORDER BY A) groups
FROM test) b WHERE a.A=b.A AND b.groups=1)