Get active users by month - mysql

Question

Using MySQL, I'm trying to get the number of active users I have in any given month. I have a table with ActivationDate and TerminationDate columns, and if the month being counted is after the ActivationDate and TerminationDate is null, then the user is active and should be counted. I would like to summarize these amounts by month. I'm thinking I could just sum each side and calculate the total but breaking that down won't give me a running total. I've tried with window functions, but I don't have enough experience with them to know exactly what I'm doing wrong and I'm not certain how to ask the right question.
So for instance, if I have the following data...
UserId ActivationDate TerminationDate
1 2020-01-01 null
2 2020-01-15 null
3 2020-01-20 2020-01-30
4 2020-02-01 null
5 2020-02-14 2020-02-27
6 2020-02-15 2020-02-28
7 2020-03-02 null
8 2020-03-05 null
9 2020-03-20 2020-03-21
I would like my results to be similar to:
2020-01 2 (there are 2 active users, since one signed up but cancelled before the end of the month)
2020-02 3 (2 from the previous month, plus 1 that signed up this month and is still active)
2020-03 5 (3 from previous, 2 new, 1 cancellation)

Answer 1

You can unpivot, then aggregate and sum. In MySQL 8.0.14 or higher, you can use a lateral join:
select date_format(x.dt, '%Y-%m-01') as dt_month,
sum(sum(cnt)) over(order by date_format(x.dt, '%Y-%m-01')) as cnt_active_users
from mytable t
cross join lateral (
select t.activationdate as dt, 1 as cnt
union all select t.terminationdate, -1
) x
where x.dt is not null
group by dt_month
order by dt_month
In earlier 8.x versions:
select date_format(x.dt, '%Y-%m-01') as dt_month,
sum(sum(cnt)) over(order by date_format(x.dt, '%Y-%m-01')) as cnt_active_users
from (
select activationdate as dt, 1 as cnt from from mytable
union all select terminationdate, -1 from mytable
) x
where x.dt is not null
group by dt_month
order by dt_month

Answer 2

You don't say what version of MySQL. If you're using 8.0, this should work:
create table userdates (
UserId int not null,
ActivationDate date not null,
TerminationDate date null
);
insert into userdates (UserId, ActivationDate, TerminationDate)
values
(1, cast("2020-01-01" as date), null )
, (2, cast("2020-01-15" as date), null )
, (3, cast("2020-01-20" as date), cast("2020-01-30" as date))
, (4, cast("2020-02-01" as date), null )
, (5, cast("2020-02-14" as date), cast("2020-02-27" as date))
, (6, cast("2020-02-15" as date), cast("2020-02-28" as date))
, (7, cast("2020-03-02" as date), null )
, (8, cast("2020-03-05" as date), null )
, (9, cast("2020-03-20" as date), cast("2020-03-21" as date))
, (10, cast("2020-07-20" as date), null)
, (11, cast("2019-09-12" as date), cast("2019-09-14" as date));
WITH RECURSIVE d (dt)
AS (
SELECT cast("2019-01-01" as date)
UNION ALL
SELECT date_add(dt, interval 1 month)
FROM d
WHERE dt < cast("2020-12-01" as date)
)
select d.dt
, count(distinct ud.UserId) as UserCount
from userdates ud
right outer join d on d.dt >= date_format(ud.ActivationDate, '%Y-%m-01')
and (d.dt <= ud.TerminationDate or ud.TerminationDate is null)
group by d.dt;