Problems computing dates within a given range based on conditions

Question

Hello. I'm working on a part of a scheduling app and am having a problem with this one section. I tried doing some searches on this forum, but no one seems to be trying to compute anything similar to what I'm doing.
Basically, I have a select list with the values: "Every", "Every Other", "Every 1st", "Every 2nd", "Every 3rd", and "Every 4th". I also have a checkbox with the values for the days of the week, from Sunday to Saturday. What I'm trying to do is that if I have a start and an end date, how to compute which days (from the various combinations of the select list and checkbox items chosen) fall in this range. Are there any functions available to help with this task? Someone mentioned the next_day() function, but looking at that function it seems like this is still pretty complicated logic-wise. Is it, or am I missing something?

Answer 1

Hi,
As Etbin said, if you can get dbms_scheduler (or some other supplied code) to do all or part of your job, that's the best thing to do.
Assuming you have to produce a list of targeted run dates, here's a SQL query that does that.
The query uses four bind variables:
(1) :start_dt, a date in DD-Mon-YYY format (other formats would work as well; easily modified)
(2) :end_dt
(3) :freq_txt, one of 'Every', 'Every Other', 'Every 1st', etc.
(4) :day_txt, a comma-delimited list of days wanted, e.g. 'Tue,Thu'. (They don't have to be in order. If you're using 3-letter English abbreviations, they don't really have to be delimited.)
I assume your app can produce these from the screen inputs.
WITH     d     AS
(     -- Begin sub-query d to find all dates in range
     SELECT     TO_DATE (:start_dt, 'DD-Mon-YYYY') + LEVEL - 1     AS dt
     FROM     dual
     CONNECT BY     LEVEL <= TO_DATE (:end_dt, 'DD-Mon-YYYY')
                    + 1
                    - TO_DATE (:start_dt, 'DD-Mon-YYYY')
)     -- End sub-query d to find all dates in range
,     w     AS
(     -- Being sub-query w to compute week_num
     SELECT     dt
     ,     1 + FLOOR ((dt - TRUNC (dt, 'MM')) / 7)     AS week_num
     FROM     d
     WHERE     :day_txt     LIKE '%' || TO_CHAR (dt, 'Dy') || '%'
)     -- End sub-query w to compute week_num
SELECT     TO_CHAR (dt, 'DD-Mon-YYYY Dy')     AS run_dt
,     week_num
FROM     w
WHERE     (:freq_txt = 'Every')
OR     (:freq_txt = 'Every Other'     AND     week_num IN (1, 3, 5))
OR     (:freq_txt = 'Every 1st'     AND     week_num = 1)
OR     (:freq_txt = 'Every 2nd'     AND     week_num = 2)
OR     (:freq_txt = 'Every 3rd'     AND     week_num = 3)
OR     (:freq_txt = 'Every 4th'     AND     week_num = 4)
ORDER BY     dt
;If you don't understand what the subqueries are doing, run them alone. (e.g. "WITH d AS (...) SELECT * FROM d;")