SQL Count distinct per 30 days - sql

Question

Can SQL distinct count per 30 days backward or MAU (Monthly active user)? for example if I have data like this:
date user
1/1/2020 A
1/2/2020 B
1/2/2020 C
...
1/30/2020 Z
And I transform it into like this using DISTINCT COUNT
date distinct_user
1/1/2020 1
1/2/2020 2
...
1/30/2020 30
To make it easier, assume that distinct user is the number of distinct users that active per days and there is no overlap between days (in reality there is overlap). So the result of MAU will be like this
date distinct_user MAU
1/1/2020 1 1
1/2/2020 2 3
...
1/30/2020 30 465
465 is the result of calculating distinct user in 30 days (with assumption no overlap user every days). so if there is 5 new user that active on 1/31/2020, the result will be like this
date distinct_user MAU
1/1/2020 1 1
1/2/2020 2 3
...
1/30/2020 30 465
1/31/2020 5 469
469 is from (Last MAU) + (new distinct user) - (distinct user from 1/1/2020 because the range is 30 days) so the result is 465 + 5 - 1 with the assumption that 5 users that active on 1/31/2020 is not active from 1/2/2020 to 1/30/2020

Answer 1

There are different approches to answer this question, the better one in terms of performance may be the following :
SELECT mt1.`date`, SUM(mt2.distinct_user) AS MAU
FROM (
SELECT `date`
FROM myTable
GROUP BY `date`
) mt1 INNER JOIN (
SELECT `date`, SUM(distinct_user) AS distinct_user
FROM myTable
GROUP BY `date`
) mt2
WHERE mt2.`date` BETWEEN mt1.`date` - INTERVAL 29 DAY AND mt1.`date`
GROUP BY mt1.`date`
ORDER BY mt1.`date`;
SEE DEMO HERE

Answer 2

Perhaps the simplest method is to "unpivot" the data and reaggregate:
with t1 as (
select date, user, 1 as inc
from t
union all
select date + interval 30 day, user, -1 as inc
from t
),
select date,
sum(case when sum_inc > 0 then 1 else 0 end) as running_30day_users
from (select t1.*,
sum(inc) over (partition by user order by date) as sum_inc
from t1
) t1
group by date;
I should note that this can also be expressed in SQL as:
select distinct date, running_30
from (select t.*,
count(distinct user) over (order by date range between interval 29 day preceding and current date) as running_30
from t
) t;
However, I'm not sure if Athena supports that syntax.