Show next date based on criteria - mysql

Question

In perl I have the following database query:
my $list = $db->SelectARef("SELECT p.*, u.usr_login, u.usr_money, u.usr_email, u.usr_pay_email, u.usr_pay_type
FROM Payments p, Users u
WHERE status='PENDING'
AND p.usr_id=u.usr_id
ORDER BY u.usr_pay_type");
Within the array result there is a field named "created".
What I want to do is add another element to the array for each row as "next payment".
Payments are processed after 30 full days from the datetime value but only on the 6th day of every month. Basically I want each result to have a "next payment" element stating which day and month they should get paid on.
e.g created = 2013-07-29 18:55:37
30 days from this is the 28th August 2013
Therefore the next payment date would be the 6th September
I have no idea where to start with this, any help anyone can provide would be greatly appreciated!
Thanks

Answer 1

The following shows the logic:
select (date(t) - interval (day(t) - 1) day) + interval 1 month + interval 5 day
from (select cast(now() as datetime) as t) t;
You can put this into your query as:
SELECT p.*, u.usr_login, u.usr_money, u.usr_email, u.usr_pay_email, u.usr_pay_typel,
created - interval (day(created) - 1) day) + interval 1 month + interval 5 day as nextpay
FROM Payments p join
Users u
on p.usr_id=u.usr_id
WHERE status='PENDING'
ORDER BY u.usr_pay_type;
The expression can be simplified to:
created - interval (day(created) + 4) day) + interval 1 month as nextpay

Answer 2

If I understand correctly, you want to compute a next_payment date which is the nearest 6th day of a month that is at least 30 days from the created date. Another way of saying that is that your due date is always the 6th of the month with at least a 30 day grace period from the close date.
Unfortunately, we can't do a simple rounding or stepping here, because the Gregorian calendar isn't quite a regular series. (It quite isn't a regular series?) So, we'll put conditional logic into a query. As this looks just awful, we'll hide it in a function:
DELIMITER //
CREATE FUNCTION next_payment(close_date DATE)
RETURNS DATE
DETERMINISTIC
READS SQL DATA
LANGUAGE SQL
BEGIN
DECLARE min_grace INT DEFAULT 30; -- minimum grace period of 30 days
DECLARE bill_dom INT DEFAULT 6; -- billing day of month is the 6th
DECLARE d DATE;
SET d = close_date + INTERVAL min_grace DAY;
IF DAY(d) > bill_dom THEN -- Did 30 days\' grace put us beyond the 6th?
SET d = d + INTERVAL 1 MONTH; -- If so, advance to next month.
END IF;
RETURN d + INTERVAL(bill_dom - DAY(d)) DAY; -- Now "round" to the 6th of the month
END
//
DELIMITER ;
How you use the logic is up to you. Adding SELECT next_payment(p.created) AS "next_payment", ... will give you the additional column you want. You could parameterize the above (e.g., CREATE FUNCTION next_payment(close_date DATE, min_grace INT, bill_dom INT)) for greater flexibility. You might rip the SET/IF logic out of the function and jam it into one unwieldy CASE/END statement in your SELECT. You could create a VIEW that automatically "appends" a next_payment column to the table, or ALTER TABLE and populate a new column, etc.