Question

I need to get the date from date table minus 3 days (business days to be specific). Below is the structure:

      date           business day
      1/1/13         Y  
      1/2/13         N
      1/3/13         Y
      1/4/13         Y
      1/5/13         Y

Suppose if todays date is 1/5/13 then I need to get current_date - 3(business)days which is 1/1/13 (since 1/2/13 is a non – business day)

Answer 1

This can be done with what is known as a ‘windowing’ function. The following should work on all (recent) versions of DB2 (untested, though, as I’m away from my instance):

SELECT calendar_date
FROM (SELECT calendar_date, ROW_NUMBER() OVER(ORDER BY calendar_date DESC) as rownum
      FROM Calendar
      WHERE calendar_date < CURRENT_DATE
            AND business_day = 'Y') c
WHERE ROW_NUM = 3

Which should result in 2013-01-01.

If you are doing a lot of similar math, I’d actually recommend creating stored procedures for this sort of thing, say something like addBusinessDays(calendar_date, days_to_add). Although you have to access a table for it to work, you should be able to mark it as DETERMINISTIC (well, assuming the compiler lets you get away with that, and that days aren’t suddenly switched…)