I need to generate a series of months within a select - postgresql

Question

I work with PostgreSQL integrated into Redash, and am currently needing to do the following report:
generate active_month for each subscription in the database, following the rule: for each month in the last 12, if that month falls in between the subscription's creation and cancelation date, create a line with that month as 'active_month'
report should contain count of subscriptions on each active_month, filtered by unit
My current code uses a nested select and looks like the following:
select
unit,
active_month,
subscriptions
from
(
select
unit_id unit,
to_char(
date_trunc('month', created_at) :: timestamp :: date,
'YYYY-MM'
) active_month,
count(id) subscriptions
from
[DATABASE HERE]
where
created_at >= (timestamp '{{ today }}' - interval '12 months')
group by
created_at,
unit
order by
active_month,
unit_id
) inner_query
group by
active_month,
unit,
subscriptions
order by
active_month,
unit
However, this doesn't work, as:
It only takes into consideration the creation month, and not every month for which the subscription was active
It seems to be counting different dates from the same month as different fields, since there are multiple lines with the same unit and month on them
Any suggestions about how can I fix this?

Answer 1

Generate the Date Range for the months you are interested in (With CTE or sub-select) then use the element contained operator for appropriate date selection. See example, of course I just generated random data and it is dependent on my actually understanding what you are asking. Which is questionable!
with calendar (date_range) as
( select daterange ( (current_date-interval '12 months')::date
, current_date
, '[]'
)
) --select * calendar;
select
unit,
active_month,
subscriptions
from
(
select
unit_id unit,
to_char(
date_trunc('month', created_at) :: timestamp :: date,
'YYYY-MM'
) active_month,
count(id) subscriptions
from
DATABASE_HERE
where
created_at::date <# (select date_range from calendar) --<<<< change
group by
created_at,
unit
order by
active_month,
unit_id
) inner_query
group by
active_month,
unit,
subscriptions
order by
active_month,
unit;