SET NOCOUNT ON;
DECLARE @t TABLE (st_no INT, year INT, month INT, sal MONEY)
INSERT INTO @t (st_no, year, month, sal) SELECT 1,2008,1,1000
INSERT INTO @t (st_no, year, month, sal) SELECT 1,2008,2,1000
INSERT INTO @t (st_no, year, month, sal) SELECT 1,2008,3,1000
INSERT INTO @t (st_no, year, month, sal) SELECT 1,2008,4,1010
INSERT INTO @t (st_no, year, month, sal) SELECT 1,2008,5,1010
INSERT INTO @t (st_no, year, month, sal) SELECT 1,2008,6,1010
INSERT INTO @t (st_no, year, month, sal) SELECT 1,2008,7,1010
INSERT INTO @t (st_no, year, month, sal) SELECT 1,2008,8,1020

;WITH cte AS (
	SELECT *, 
		ROW_NUMBER() OVER(PARTITION BY sal ORDER BY year, month, sal) AS seq
	FROM @t
)
SELECT year, month, sal, seq 
FROM cte
WHERE seq = 1

/*
year        month       sal                   seq
----------- ----------- --------------------- --------------------
2008        1           1000.00               1
2008        4           1010.00               1
2008        8           1020.00               1
*/

WITH cte AS (
        SELECT *, 
                ROW_NUMBER() OVER(PARTITION BY st_no ORDER BY year, month) AS seq
        FROM @t
)
SELECT a.st_no, a.year, a.month, a.sal
FROM cte a
WHERE NOT EXISTS(SELECT * FROM cte b
                 WHERE a.st_no=b.st_no
                   AND a.sal=b.sal
                   AND a.seq=b.seq+1)