DECLARE @RepStartTime datetime, @RepEndTime datetime, @RepFreq int


SET @RepStartTime = '2001-01-01 00:00:00'
SET @RepEndTime = '2002-01-01 00:00:00'
SET @RepFreq = 3600


SELECT HStart,
  @RepFreq - SUM(
    CASE WHEN InTime IS NULL THEN
      0
    ELSE
      DATEDIFF(second,
        CASE WHEN OutTime > HStart THEN OutTime ELSE HStart END,
        CASE WHEN InTime < HEnd THEN InTime ELSE HEnd END)
    END
  )
FROM (
  SELECT HStart, DATEADD(second, @RepFreq, HStart) HEnd
  FROM (
    SELECT DATEADD(second, N * @RepFreq, @RepStartTime) HStart
    FROM Numbers
    WHERE N * @RepFreq < DATEDIFF(second, @RepStartTime, @RepEndTime)
  ) a
) hours
LEFT JOIN (
  SELECT OutTime, InTime
  FROM (
    SELECT T1.LogoutTime OutTime, MIN(T2.LoginTime) InTime
    FROM tblLoginLogout AS T1, tblLoginLogout AS T2
    WHERE T1.LogoutTime < T2.LoginTime
    GROUP BY T1.LogoutTime) a
  WHERE NOT EXISTS (
    SELECT 1 FROM tblLoginLogout
    WHERE LoginTime < InTime AND LogoutTime > OutTime)
) lapses ON HStart < InTime AND HEnd > OutTime
GROUP BY HStart
ORDER BY HStart

CREATE TABLE tblLoginLogout (
  Operator int NOT NULL,
  LoginTime datetime NOT NULL,
  LogoutTime datetime NOT NULL)

SET NOCOUNT ON
DECLARE @o int, @shiftstart datetime
DECLARE @start datetime, @end datetime


SET @shiftstart = '2001-01-01 00:00:00'

WHILE @shiftstart < '2002-01-01 00:00:00'
BEGIN
  SET @o = 0
  WHILE @o < 3
  BEGIN
    SET @start = DATEADD(second, FLOOR(RAND() * 3600.0 - 1800.0), @shiftstart)
    SET @shiftstart = DATEADD(hour, 8, @shiftstart)
    SET @end = DATEADD(second, FLOOR(RAND() * 3600.0 - 1800.0), @shiftstart)
    INSERT INTO tblLoginLogout VALUES (@o, @start, @end)
    SET @o = @o + 1
  END
END
SET NOCOUNT OFF