CREATE FUNCTION dbo.fnTimeOverlap
(
	@FromTime DATETIME,
	@ToTime DATETIME,
	@Login DATETIME,
	@Logout DATETIME
)
RETURNS INT
AS

BEGIN
	DECLARE	@Temp DATETIME,
		@Seconds INT

	IF @FromTime > @ToTime
		SELECT	@Temp = @FromTime,
			@FromTime = @ToTime,
			@ToTime = @Temp

	IF @Login > @Logout
		SELECT	@Temp = @Login,
			@Login = @Logout,
			@Logout = @Temp

	SELECT	@Seconds = 	CASE
					WHEN @FromTime <= @Login AND @Login <= @ToTime AND @ToTime <= @Logout THEN DATEDIFF(second, @Login, @ToTime)
 					WHEN @FromTime <= @Login AND @Logout <= @ToTime THEN DATEDIFF(second, @Login, @Logout)
					WHEN @Login <= @FromTime AND @ToTime <= @Logout THEN DATEDIFF(second, @FromTime, @ToTime)
					WHEN @Login <= @FromTime AND @FromTime <= @Logout AND @Logout <= @ToTime THEN DATEDIFF(second, @FromTime, @Logout)
				END

	RETURN	@Seconds
END

create function dbo.fntimeoverlap_2005
(
	@FromTime datetime,
	@ToTime datetime,
	@Login datetime,
	@Logout datetime
)
returns int
as
begin
	declare @seconds int
	
	;with t(t0,t1) as
	(
		select min(t),max(t) from
		(select @FromTime union all select @ToTime)s(t)
	)
	,s(s0,s1) as
	(
		select min(t),max(t) from
		(select @Login union all select @Logout)s(t)
	)
	select @seconds = datediff(	second
					,(select max(t) from(select t0 from t union all select s0 from s)x(t))
					,(select min(t) from(select t1 from t union all select s1 from s)y(t))	)
	where	(select s0 from s) <= (select t1 from t)
		and
		(select s1 from s) >= (select t0 from t)

	return @seconds
end
go