MySQL select unique months when discounts are valid - mysql

Question

I have a MySQL database containing discounts. A simplified version looks like this:
id | start (UNIX timestamp) | end (UNIX timestamp)
45 | 1384693200 | 1398992400
68 | 1386018000 | 1386277200
263 | 1388530800 | 1391209200
A discount can last a few days, a few months, or even a few years. I'm looking for a way to select a unique list of months where (future) discounts are valid.
If there is:
a discount which starts in november 2013 and ends in april 2014
a discount which starts in december 2013 and ends in the same month
a discount which starts in january 2014 and ends one month later
a discount which starts in june 2014 and ends the same month
The output should be:
- December (2013)
- January (2014)
- February (2014)
- March (2014)
- April (2014)
- June (2014)
November 2013 is not shown because it is in the past. May 2014 is not shown because there is no discount in that month.
Can somebody help?
Thanks in advance!

Answer 1

Create a table containing a sequence of numbers from 0 to a number of month you could ever require, and join this table to your table.
This is example how to get a list of years+months separately for each id
SELECT id,
year( start + interval x month ) year,
month( start + interval x month ) month
FROM
numbers n
JOIN
(
SELECT id,
from_unixtime( start ) start,
from_unixtime( end ) end
FROM Table1
) q
ON n.x <= period_diff( date_format( q.end, '%Y%m' ),date_format( q.start, '%Y%m' ))
ORDER BY id, year, month ;
Demo --> http://www.sqlfiddle.com/#!9/d7cfc/4
If you want to combine years+months for all id, skip id column and use GROUP BY
SELECT year( start + interval x month ) year,
month( start + interval x month ) month
FROM
numbers n
JOIN
(
SELECT id,
from_unixtime( start ) start,
from_unixtime( end ) end
FROM Table1
) q
ON n.x <= period_diff( date_format( q.end, '%Y%m' ),date_format( q.start, '%Y%m' ))
GROUP BY year, month
ORDER BY year, month ;
If you want to skip past years and months, add WHERE year >= current year AND month >= current month, this is a trivial change. Also add another WHERE end < current-unix-time in the subquery to filter out unwanted past rows.