DECLARE @sessionsCount int = 1;
DECLARE @timeLimitBetweenSessions int = 300;
IF OBJECT_ID('tempdb..#TestTable') IS NOT NULL DROP TABLE #TestTable
SELECT
		a.Id, a.User_Id UserId1,
		b.User_Id UserId2,
		a.CreateDateTime StartActivity,
		b.CreateDateTime EndActivity,
		a.RowNum1,
		b.RowNum2,
		@sessionsCount as 'SessionNumber'
  INTO #TestTable
  FROM (
  select Id,
		User_Id ,
		CreateDateTime
		,ROW_NUMBER() OVER (ORDER BY [User_Id], [CreateDateTime]) AS RowNum1
  from  [dbo].[PageLoadings]) as a
  JOIN (
  SELECT [Id]
      ,[User_Id]
      ,[CreateDateTime]
	  ,ROW_NUMBER() OVER (ORDER BY [User_Id], [CreateDateTime]) AS RowNum2
	    FROM [dbo].[PageLoadings]) as b
		on a.RowNum1 = (b.RowNum2 - 1)
	UPDATE #TestTable SET
		SessionNumber =
		CASE WHEN UserId1 <> UserId2 THEN 0
		ELSE CASE WHEN DATEDIFF(SECOND, StartActivity, EndActivity) > @timeLimitBetweenSessions THEN 0
			ELSE @sessionsCount
			END
		END,
		@sessionsCount =
		CASE WHEN UserId1 <> UserId2 THEN @sessionsCount + 1
		ELSE
			CASE WHEN DATEDIFF(SECOND, StartActivity, EndActivity) > @timeLimitBetweenSessions THEN @sessionsCount + 1
			ELSE @sessionsCount
			END
		END;
	--Костыль для последней строки на случай, когда между последней и предпоследней сессий прошло > 5 минут
	WITH LastRecord AS
	(
	SELECT *
	FROM #TestTable
	WHERE RowNum2 = (SELECT MAX(RowNum2) FROM #TestTable)
	)
	UPDATE LastRecord SET SessionNumber =
		CASE WHEN SessionNumber = 0 THEN @sessionsCount
			ELSE 0
			END,
		StartActivity =
		CASE WHEN SessionNumber = 0 THEN EndActivity
			ELSE StartActivity
			END;
	UPDATE #TestTable SET EndActivity = DATEADD(minute, 5, EndActivity)
		WHERE EndActivity IN (SELECT max(EndActivity)
		FROM #TestTable
		WHERE SessionNumber <> 0
		GROUP BY SessionNumber)
	SELECT UserId2 as UserId, min(StartActivity) as StartActivity, max(EndActivity) as EndActivity FROM #TestTable
		WHERE SessionNumber <> 0
		GROUP BY SessionNumber, UserId2