SQL Server 2012

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CREATE TABLE DBO.PRUEBA
(
    NUM_GROUP INT NOT NULL,
    NUM_ORDER INT NOT NULL,
    NUM_FLAG INT NOT NULL,
    PRIMARY KEY (NUM_GROUP, NUM_ORDER)
);
INSERT INTO DBO.PRUEBA 
(NUM_GROUP, NUM_ORDER, NUM_FLAG)
VALUES
  (1111, 1, 0)
 ,(1111, 2, 0)
 ,(1111, 3, 1)
 ,(1111, 4, 1)
 ,(1111, 5, 1)
 ,(1111, 6, 1)
 ,(1111, 7, 1)
 ,(1111, 8, 0)
 ,(1111, 9, 1)
 ,(1111, 10, 1)
 ,(1111, 11, 0)
 ,(1111, 12, 0)
 ,(1111, 13, 0)
 ,(6666, 1, 0)
 ,(6666, 2, 0)
 ,(6666, 3, 1)
 ,(6666, 4, 1)
17 rows affected

IF OBJECT_ID('tempdb..#tmpPRUEBA', 'U') IS NOT NULL
    DROP TABLE #tmpPRUEBA; 

CREATE TABLE #tmpPRUEBA
(
    NUM_GROUP INT NOT NULL,
    RN_GROUP INT NOT NULL,
    NUM_ORDER INT NOT NULL,
    NUM_FLAG INT NOT NULL,
    PRIMARY KEY (NUM_GROUP, RN_GROUP)
);
INSERT INTO #tmpPRUEBA
(NUM_GROUP, NUM_ORDER, NUM_FLAG, RN_GROUP)
SELECT NUM_GROUP, NUM_ORDER, NUM_FLAG
, ROW_NUMBER() OVER (
      PARTITION BY NUM_GROUP 
      ORDER BY NUM_ORDER) AS RN_GROUP
FROM DBO.PRUEBA;
17 rows affected

SELECT 
t1.NUM_GROUP, 
t1.NUM_ORDER, 
t1.NUM_FLAG,
SUM(CASE 
    WHEN t1.NUM_FLAG = t2.NUM_FLAG 
    THEN 0 
    ELSE 1 
    END)
    OVER (PARTITION BY t1.NUM_GROUP 
          ORDER BY t1.RN_GROUP) AS [Rank]
FROM #tmpPRUEBA t1
LEFT JOIN #tmpPRUEBA t2
  ON t2.NUM_GROUP = t1.NUM_GROUP
 AND t2.RN_GROUP = t1.RN_GROUP - 1;


NUM_GROUP
NUM_ORDER
NUM_FLAG
Rank


1111
1
0
1

1111
2
0
1

1111
3
1
2

1111
4
1
2

1111
5
1
2

1111
6
1
2

1111
7
1
2

1111
8
0
3

1111
9
1
4

1111
10
1
4

1111
11
0
5

1111
12
0
5

1111
13
0
5

6666
1
0
1

6666
2
0
1

6666
3
1
2

6666
4
1
2

NUM_GROUP	NUM_ORDER	NUM_FLAG	Rank
1111	1	0	1
1111	2	0	1
1111	3	1	2
1111	4	1	2
1111	5	1	2
1111	6	1	2
1111	7	1	2
1111	8	0	3
1111	9	1	4
1111	10	1	4
1111	11	0	5
1111	12	0	5
1111	13	0	5
6666	1	0	1
6666	2	0	1
6666	3	1	2
6666	4	1	2