SQL Server 2016

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CREATE TABLE TEST
(
DTTM DATETIME, 
SESSION_ID INT
)

INSERT INTO TEST (DTTM, SESSION_ID)
select '2016-08-29 14:24:28.450', 297692378 union
select '2017-04-13 23:54:53.760', 297692378 union
select '2017-04-13 23:59:53.477', 297692378 union
select '2017-04-14 00:04:52.897', 297692378 union
select '2017-04-14 00:04:53.790', 297692378 union
select '2017-04-14 00:44:53.790', 297692378
6 rows affected

ALTER TABLE TEST ADD [RANK] int;
SELECT DTTM, SESSION_ID,
       DATEDIFF(minute, COALESCE(LAG(DTTM) OVER (ORDER BY DTTM, SESSION_ID), DTTM), DTTM) DIF_MIN
FROM   TEST


DTTM
SESSION_ID
DIF_MIN


29/08/2016 14:24:28
297692378
0

13/04/2017 23:54:53
297692378
327450

13/04/2017 23:59:53
297692378
5

14/04/2017 00:04:52
297692378
5

14/04/2017 00:04:53
297692378
0

14/04/2017 00:44:53
297692378
40


DECLARE @dttm datetime,
        @session_id int,
        @diff_min int,
        @acm_diff int,
        @rank int,
        @last_dttm datetime;

SET @diff_min = 0;
SET @acm_diff = 0;
SET @rank = 0;
SET @last_dttm = NULL;

DECLARE curMin CURSOR FAST_FORWARD  FOR
SELECT DTTM, SESSION_ID,
       DATEDIFF(minute, COALESCE(LAG(DTTM) OVER (ORDER BY DTTM, SESSION_ID), DTTM), DTTM) DIF_MIN
FROM   TEST

OPEN curMin;
FETCH NEXT FROM curMin INTO @dttm, @session_id, @diff_min;

WHILE @@FETCH_STATUS = 0  
BEGIN
    IF @last_dttm IS NULL OR @acm_diff + @diff_min > 20
    BEGIN
        SET @rank = @rank + 1;
        SET @acm_diff = 0;
    END
    ELSE
    BEGIN
        SET @acm_diff = @acm_diff + @diff_min;
    END
    
    UPDATE TEST
    SET    [RANK] = @rank
    WHERE  DTTM = @dttm
    AND    SESSION_ID = @session_id;
    
    SET @last_dttm = @dttm;
    
    FETCH NEXT FROM curMin INTO @dttm, @session_id, @diff_min;
END

CLOSE curMin;

SELECT   DTTM, SESSION_ID,
         DATEDIFF(minute, DTTM, COALESCE(LEAD(DTTM) OVER (ORDER BY DTTM, SESSION_ID), DTTM)) DIF_MIN,
         [RANK]
FROM     TEST 
ORDER BY DTTM, SESSION_ID;


DTTM
SESSION_ID
DIF_MIN
RANK


29/08/2016 14:24:28
297692378
327450
1

13/04/2017 23:54:53
297692378
5
2

13/04/2017 23:59:53
297692378
5
2

14/04/2017 00:04:52
297692378
0
2

14/04/2017 00:04:53
297692378
40
2

14/04/2017 00:44:53
297692378
0
3

DTTM	SESSION_ID	DIF_MIN
29/08/2016 14:24:28	297692378	0
13/04/2017 23:54:53	297692378	327450
13/04/2017 23:59:53	297692378	5
14/04/2017 00:04:52	297692378	5
14/04/2017 00:04:53	297692378	0
14/04/2017 00:44:53	297692378	40