Postgres cumulative count over time

Question

I'm trying to find the number of active subscriptions in a month over a timescale.

Table: Subscriptions Fields:

• id
• "creationDate"
• "deletionDate"

A subscription is considered active at a particular timestamp when:

1. deletionDate is null
2. the particular timestamp falls between creationDate and deletionDate

Example:

• Subscription A has creationDate "2014-06-27 11:37:34.205+00" and deletionDate "2014-08-01 04:16:34.435+00". Subscription A is considered active in June 2014, July 2014, and Aug 2014.
• Subscriptions B has creationDate "2014-06-27 11:37:34.205+00" and deletionDate "2014-06-28 11:37:34.205+00". Subscription B is only active in June 2014.
• Subscription C has creationDate "2014-06-27 11:37:34.205+00" and no deletionDate. Subscription C is considered active for months June 2014 onwards till the current month.

This is what I tried:

select "Month", sum(sub) over (order by "Month" asc) as "Active subscriptions"
from
(select to_char(subscriptions."creationDate" at time zone '-7', 'YYYY-MM') as "Month", 
    count(distinct subscriptions.id) as sub
    from subscriptions
    where (to_char(subscriptions."deletionDate" at time zone '-7', 'YYYY-MM') is null 
        or to_char(subscriptions."deletionDate" at time zone '-7', 'YYYY-MM') >= to_char(subscriptions."creationDate" at time zone '-7', 'YYYY-MM') )
    group by "Month") as foo

However, the issue with this is that it is including the count of non-active subscriptions in the previous month. To illustrate what I mean, my query above seems to include Subscription B (in example above) as an active subscription in July 2014.

I'm not sure how to get my "Active subscriptions" for a particular month to remove the count of subscriptions that are no longer active in the previous months.

Thank you! :)

Answer 1

SELECT m, count(subscriptions.*) 
FROM subscriptions 
JOIN generate_series('2010-01-01'::date, now(), interval '1 mon') AS m
ON m >= subscriptions.creationDate AND 
       (subscriptions.deletionDate IS NULL OR m <= subscriptions.deletionDate)
/* You may get better indexed performance if you use a date 
   far in the future for current
  accounts, instead of NULL. Then you can use BETWEEN */
GROUP BY m ORDER BY m;