Question

I am trying to write a function in mySQL that takes two dates(startDate and endDate) as parameters. It then calculates the days in each month.

The database contains a targetRevenue table that has got the target revenue values for each month and year.

id  month   year  targetRev
25    1       2012    1000.00
26    2       2012    5000.00
27    3       2012    8000.00

The function finds the revenue for a month based on the number of days in it and then returns the total.

Example : startDate : 2012-01-19 endDate : 2012-03-24
Function returns [ targetRev(19 days in Jan) + targetRev(29 days Feb) + targetRev(24days in March)]

I am new to writing functions in mysql , so a little bit of help to get me started would be very useful. Thanks in advance!

Answer 1

If instead of your month and year columns, you represented the month of each record in your targetRevenue table by a DATE column containing the first day of each month:

ALTER TABLE targetRevenue
  ADD COLUMN first DATE;

UPDATE targetRevenue
  SET first = STR_TO_DATE(CONCAT_WS('-', year, month, 1), '%Y-%c-%e');

ALTER TABLE targetRevenue
  DROP COLUMN year,
  DROP COLUMN month;

You could then obtain the total target revenue for your project (assuming it is inclusive of both start and end date) with:

-- calculate the summation of
SELECT SUM(CONVERT(

         -- number of project days in month...
         GREATEST(0,
           -- ...is calculated as the difference between...
           DATEDIFF(
             -- ...the last day of the project in this month...
                LEAST('2012-03-24', LAST_DAY(first)),
             -- ...and the first day of the project in this month...
             GREATEST('2012-01-19', first)
           )
           -- ...plus one because first and last project days were inclusive
           + 1
         )

         -- multiply by the target revenue for this month
         * targetRev

         -- divide by the number of days in the month
         / DAY(LAST_DAY(first)),

         -- convert result to fixed-point format, to two d.p.
         DECIMAL(11,2)

       )) AS total

FROM   targetRevenue

-- only perform for months in which the project was active
WHERE  '2012-01-19' <= LAST_DAY(first) AND first <= '2012-03-24'

See it on sqlfiddle.

If you can’t change the schema, you could replace references to first with the value to which that column was updated above.