DB2 Stored Procedure Calculate Days per month dynamically - stored-procedures

Question

How to write the DB2 stored procedure to calculate the DPM logic.
For example, between Jan 1, 2014 to Sept 30, 2014 – 9 records will be displayed (one record per month). For each record, DPM (days per month) needs to be calculated at runtime, dynamically and updated in the formula :
select (((dpm*24) / 2400) * 123) / dpm
from xxx-table
where date between '2014-09-01' and '2014-01-01'
How to ensure that the correct DPM gets updated in the above mentioned formula
Expected Output
Jan - (((31*24) / 2400) * 123) / 31
Feb -(((28*24) / 2400) * 123) / 28
March - (((31*24) / 2400) * 123) / 31
April - (((30*24) / 2400) * 123) / 30
May - (((31*24) / 2400) * 123) / 31
June - (((30*24) / 2400) * 123) / 30
July - (((31*24) / 2400) * 123) / 31
August - (((31*24) / 2400) * 123) / 31
September - (((30*24) / 2400) * 123) / 30

Answer 1

As #mustaccio said, your formula always simplifies down to 1.23, by moving the first dpm multiplication "out" of the first fraction:
(dpm * (24/2400) * 123) / dpm
Then, the dpm's cancel each other out, only leaving
(24/2400) * 123
Which is the constant 1.23
However, if you really do need a days per month calculation, you could use a recursive query to build out your list of months, and then use the LAST_DAY combined with the DAY scalar functions. I'm assuming you're using DB2 for Linux/Unix/Windows, and I think LAST_DAY was added in 9.7
WITH DATES (DTE, LVL) AS (
SELECT CAST(#beginDate AS DATE), 0
FROM SYSIBM.SYSDUMMY1
UNION ALL
SELECT DTE + 1 MONTHS, LVL +1
FROM SYSIBM.SYSDUMMY1, DATES
WHERE DTE + 1 MONTHS <= #endDate
)
SELECT DAY(LAST_DAY(DTE))
FROM DATES