Tomalak posted an excellent response to an existing SO question:
http://stackoverflow.com/questions/593898/sql-datediff-advanced-usage/594087#594087
It almost worked for me but I needed to compute the business hours difference between two dates, excluding weekends, even if less than a week had elapsed. My solution adds a while loop (which is probably a bit naive, open to suggestions!) and adds a check for holidays in a holiday lookup table.
Edit: I'm not looking for this to be answered in the traditional sense, just wanted to get back to it easily and open it up for critiquing for those who may come across it in the future.
ALTER FUNCTION dbo.udfDateDiffBusinessHours (
@date1 DATETIME,
@date2 DATETIME
) RETURNS DATETIME AS
BEGIN
DECLARE @sat INT
DECLARE @sun INT
DECLARE @workday_s INT
DECLARE @workday_e INT
DECLARE @basedate1 DATETIME
DECLARE @basedate2 DATETIME
DECLARE @calcdate1 DATETIME
DECLARE @calcdate2 DATETIME
DECLARE @iteratordate DATETIME
DECLARE @cworkdays INT
DECLARE @coffdays INT
DECLARE @returnval INT
SET @workday_s = 480 -- work day start: 8 hours
SET @workday_e = 1080 -- work day end: 18 hours
-- calculate Saturday and Sunday dependent on SET DATEFIRST option
SET @sat = CASE @@DATEFIRST WHEN 7 THEN 7 ELSE 7 - @@DATEFIRST END
SET @sun = CASE @@DATEFIRST WHEN 7 THEN 1 ELSE @sat + 1 END
SET @calcdate1 = @date1
SET @calcdate2 = @date2
-- @date1: assume next day if start was after end of workday
SET @basedate1 = DATEADD(dd, 0, DATEDIFF(dd, 0, @calcdate1))
SET @calcdate1 = CASE WHEN DATEDIFF(mi, @basedate1, @calcdate1) > @workday_e
THEN @basedate1 + 1
ELSE @calcdate1
END
-- @date1: if Saturday or Sunday, make it next Monday
SET @basedate1 = DATEADD(dd, 0, DATEDIFF(dd, 0, @calcdate1))
SET @calcdate1 = CASE DATEPART(dw, @basedate1)
WHEN @sat THEN @basedate1 + 2
WHEN @sun THEN @basedate1 + 1
ELSE @calcdate1
END
-- @date1: assume @workday_s as the minimum start time
SET @basedate1 = DATEADD(dd, 0, DATEDIFF(dd, 0, @calcdate1))
SET @calcdate1 = CASE WHEN DATEDIFF(mi, @basedate1, @calcdate1) < @workday_s
THEN DATEADD(mi, @workday_s, @basedate1)
ELSE @calcdate1
END
-- @date2: assume previous day if end was before start of workday
SET @basedate2 = DATEADD(dd, 0, DATEDIFF(dd, 0, @calcdate2))
SET @calcdate2 = CASE WHEN DATEDIFF(mi, @basedate2, @calcdate2) < @workday_s
THEN @basedate2 - 1
ELSE @calcdate2
END
-- @date2: if Saturday or Sunday, make it previous Friday
SET @basedate2 = DATEADD(dd, 0, DATEDIFF(dd, 0, @calcdate2))
SET @calcdate2 = CASE DATEPART(dw, @calcdate2)
WHEN @sat THEN @basedate2 - 0.00001
WHEN @sun THEN @basedate2 - 1.00001
ELSE @date2
END
-- @date2: assume @workday_e as the maximum end time
SET @basedate2 = DATEADD(dd, 0, DATEDIFF(dd, 0, @calcdate2))
SET @calcdate2 = CASE WHEN DATEDIFF(mi, @basedate2, @calcdate2) > @workday_e
THEN DATEADD(mi, @workday_e, @basedate2)
ELSE @calcdate2
END
-- count full work days (subtract Saturdays, Sundays and holidays)
SET @cworkdays = DATEDIFF(dd, @basedate1, @basedate2)
SET @iteratordate = @basedate1
SET @coffdays = 0
WHILE DATEDIFF(dd, @iteratordate, @basedate2) > 0
BEGIN
IF DATEPART(dw, @iteratordate) = @sat OR DATEPART(dw, @iteratordate) = @sun OR EXISTS (SELECT holidaydate FROM dbo.holidays_lu (NOLOCK) WHERE holidaydate = @iteratordate)
SET @coffdays = @coffdays + 1
SET @iteratordate = DATEADD(dd, 1, @iteratordate)
END
SET @cworkdays = @cworkdays - @coffdays
-- calculate effective duration in minutes
SET @returnval = @cworkdays * (@workday_e - @workday_s)
+ @workday_e - DATEDIFF(mi, @basedate1, @calcdate1)
+ DATEDIFF(mi, @basedate2, @calcdate2) - @workday_e
-- return duration as an offset in minutes from date 0
RETURN DATEADD(mi, @returnval, 0)
END