--创建表
CREATE TABLE dbo.Tab
    (
    ID int NOT NULL IDENTITY (1, 1),
    Y varchar(20) NOT NULL,
    X datetime NOT NULL
    )
GO
--插入数据
INSERT INTO Tab(Y, X) values('BBB', '2007-10-23 11:11:11')
INSERT INTO Tab(Y, X) values('BBB', '2007-10-23 11:11:11')
INSERT INTO Tab(Y, X) values('BBB', '2007-10-23 10:10:10')
INSERT INTO Tab(Y, X) values('AAA', '2007-10-23 12:12:12')
INSERT INTO Tab(Y, X) values('AAA', '2007-10-23 10:10:10')
INSERT INTO Tab(Y, X) values('AAA', '2007-10-23 11:11:11')
GO

--一条SQL语句实现
  SELECT ID, Y, X
FROM Tab T
WHERE (NOT EXISTS
          (SELECT 1
         FROM Tab T2
         WHERE (T2.Y = T.Y) AND (T2.X > T.X OR
               T2.X = T.X AND T2.ID > T.ID)))