进行 SQL 查询以生成累积订阅者的最简单方法是什么？

Question

I have a subscriptions table (in postgresql) with the columns subscribed_at and unsubscribed_at . I'd like to write a simple query that produces a list of the cumulative subscriber count (subscribed minus unsubscribed as of that date) for each month end.

Each row is essentially querying: if the group_by month is "January-2020" how many subscriptions have subscribed_at date with a month on or before January 2020, and a unsubscribed_at date of null or after January 2020.

I'm just not familiar enough with SQL to know the right syntax here.

Any help would be much appreciated!

Table

+----+------+---------------+-----------------+
| id | name | subscribed_at | unsubscribed_at |
+----+------+---------------+-----------------+
|  1 | John | '2020-01-08'  | null            |
|  2 | Mary | '2020-01-09'  | '2020-01-20'    |
|  3 | Jeff | '2020-01-10'  | null            |
|  4 | Bill | '2020-02-02'  | null            |
|  5 | Bob  | '2020-02-08   | '2020-02-21'    |
+----+------+---------------+-----------------+

Query

SELECT DATE_TRUNC('month', subscribed_at) as month,
COUNT(*) as subscribers
FROM subscriptions
GROUP BY 1
ORDER BY 1

Desired Output

+---------------+-------------+
|     month     | subscribers |
+---------------+-------------+
| January 2020  |           2 |
| February 2020 |           3 |
+---------------+-------------+

Where subscribers is the cumulative net subscribers at the end of that month.

Here is a SQLFiddle with the data and query: http://www.sqlfiddle.com/#!15/cd7725/1

Answer 1

You could use generate_series() to enumerate all months between the first subscription start and the latest subscription end, and then a lateral join to compute the active subscriptions count.

select d.dt, n.no_active_subscriptions
from (
    select generate_series(
        date_trunc('month', min(subscribed_at)), 
        date_trunc('month', max(unsubsribed_at)), 
        interval '1 month'
    ) dt
    from subscriptions
) d
cross join lateral (
    select count(*) no_active_subscriptions
    from subscriptions s
    where 
        s.subscribed_at < d.dt + interval '1 month' 
        and (s.unsubscribed_at >= d.dt or s.unsubscribed_at is null)
) n

Answer 2

Here is an approach. Calculate +1 on the date someone starts and -1 on the date the stop. Then do a cumulative sum.

For the end of the month, take the last record per month:

with s as (
      select dte, sum(inc) ondate, sum(sum(inc)) over (order by dte) as subs
      from subscriptions s cross join lateral
           (values (subscribed_at, 1), (unsubscribed_at, -1)
           ) v(dte, inc)
      where v.dte is not null
      group by v.dte
     )
 select distinct on (date_trunc('month', dte)) *
 from s
 order by date_trunc('month', dte), dte desc;

Here is a db<>fiddle.

This may be exactly what you want. . . or you might want to tweak it.

1. The date you stop counting might be the day after the stop. If so, add one date in the subquery.
2. If you want to report on the last day of the month, you can use the expression (date_trunc('month', dte) + interval '1 month - 1 day')::date .