mysql fill in missing dates for each specific value in column - mysql

Question

I am trying to count the number of events in the subchannel column of my l_events table between the dates 2013-10-01 and 2014-03-31 and group by specific subchannel, year and month.
Right now my results look like:
year month subchannel count(*)
2013 10 creativemornings 1
2014 3 creativemornings 2
2013 11 founderinstitute 1
I would like my results to include rows for each specific subchannel where no events occurred occurred in that month. So something like:
year month subchannel count(*)
2013 10 creativemornings 1
2013 11 creativemornings 0
2013 12 creativemornings 0
2014 1 creativemornings 0
2014 2 creativemornings 0
2014 3 creativemornings 2
2013 10 founderinstitute 0
2013 11 founderinstitute 1
2013 12 founderinstitute 0
2014 1 founderinstitute 0
2014 2 founderinstitute 0
2014 3 founderinstitute 0
I tried to join my query to my calendar table (which includes a datetime column of all relevent dates) but it didn't work. Any thoughts?
SELECT
year(l.created) as year,
month(l.created) as month,
l.subchannel,
count(*)
FROM db.l_events l
right JOIN db.calendar c
ON (date(l.created) = c.datefield)
WHERE
l.created between '2013-10-01' and '2014-03-31'
group by
l.subchannel,
year(l.created),
month(l.created)
;

Answer 1

This will do it. It creates a Cartesian product of your subchannel list and your calendar table and LEFT JOINs the l_events table to that result set.
SELECT
year(c.datefield) as year,
month(c.datefield) as month,
s.subchannel,
count(l.created)
FROM db.calendar c
CROSS JOIN (select distinct subchannel from l_events) s
LEFT JOIN db.l_events l
ON (date(l.created) = c.datefield)
AND l.subchannel = s.subchannel
Where c.datefield between '2013-10-01' and '2014-03-31'
group by
s.subchannel,
year(c.created),
month(.created)
order by
s.subchannel,
year(c.created),
month(.created)
;