SQL DateDiff advanced usage?

Question

I need to calculate the DateDiff (hours) between two dates, but only during business-hours (8:30 - 16:00, no weekends). This result will then be put into the Reaction_Time c

Answer 1

Assuming you have a reference-table of the working days (and their hours), then I would use a 3 stage approach (pseudo-sql)

(first preclude the "all in one day" trivial example, since that simplifies the logic)

 -- days that are neither the start nor end (full days)
 SELECT @FullDayHours = SUM(day start to day end)
 FROM   reference-calendar
 WHERE  Start >= midnight-after-start and End <= midnight-before-end

 -- time after the [query start] to the end of the first working day
 SELECT @FirstDayHours = [query start] to day end
 FROM   reference-calandar
 WHERE  start day

 -- time from the start of the last working day to the [query end]
 SELECT @LastDayHours = day start to [query end]
 FROM   reference-calendar
 WHERE  end-day

 IF @FirstDayHours < 0 SET @FirstDayHours = 0 -- starts outside working time
 IF @LastDayHours < 0 SET @LastDayHours  = 0 -- ends outside working time

 PRINT @FirstDayHours  + @FullDayHours + @LastDayHours

Obviously it is a bit hard to do properly without more context...