按该字段排序时,多行中相同的只取第一-----------行如何查询出这样的数据?

[雄牛]

有这样的表数据,

ID UserName Logintime
1 cowbo 2017-1-1 10:50
2 cowbo 2017-2-1 10:50
3 cowbo 2017-3-1 10:50
4 hackc 2017-3-2 10:50

如何查询出:
ID UserName Logintime
1 cowbo 2017-1-1 10:50
2017-2-1 10:50
2017-3-1 10:50
4 hackc 2017-3-2 10:50

[卖水果的net]

仅仅是username 列这样处理吗？

[学海无涯-回头是岸]

UPDATE  T2 SET ID='',UserName=''  FROM 
(
(SELECT T1.*,ROW_NUMBER()OVER(PARTITION BY UserName,ORDER BY Logintime) AS RN) T2
WHERE RN>1

[二月十六]

--测试数据
if not object_id(N'Tempdb..#T') is null
	drop table #T
Go
CREATE TABLE #T(id INT,UserName NVARCHAR(100),Logintime DATETIME)
Insert #T
select 1,N'cowbo','2017-1-1 10:50' union all
select 2,N'cowbo','2017-2-1 10:50' union all
select 3,N'cowbo','2017-3-1 10:50' union all
select 4,N'hackc','2017-3-2 10:50'
Go
--测试数据结束
;WITH tempa AS (
SELECT *,ROW_NUMBER()OVER(PARTITION BY UserName ORDER BY id) AS num FROM #T
)
SELECT  ( CASE WHEN tempa.num = 1 THEN id
               ELSE NULL
          END ) AS id ,
        ( CASE WHEN tempa.num = 1 THEN tempa.UserName
               ELSE NULL
          END ) AS UserName ,
        tempa.Logintime
FROM    tempa;