How to calculate running sums with append-only rows - sql

Question

I have a table where rows are never mutated but only inserted; they are immutable records. It has the following fields:
id: int
user_id: int
created: datetime
is_cool: boolean
likes_fruits: boolean
An object is tied to a user, and the "current" object for a given user is the one that has the latest created date. E.g. if I want to update is_cool for a user, I'd append a record with a new created timestamp and is_cool=true.
I want to calculate how many users are is_cool at the end of each day. I.e. I'd like the output table to have the columns:
day: some kind of date_trunc('day', created)
cool_users_count: number of users that have is_cool at the end of this day.
What SQL query can i write that does this? FWIW I'm using Presto (or Redshift if need to).
Note that there are other columns, e.g. likes_fruits, which means a record where is_cool is false does not mean is_cool was just changed to false - it could have been false for a while.
This is what procedural pseudo-code would look like to represent what I'd want to do in SQL:
// rows = ...
min_date = min([row.created for row in rows])
max_date = max([row.created for row in rows])
counts_by_day = {}
for date in range(min_date, max_date):
rows_up_until_date = [row for row in rows if row.created <= date]
latest_row_by_user = rows_up_until_date.reduce(
{},
(acc, row) => acc[row.user_id] = row,
)
counts_by_day[date] = latest_row_by_user.filter(row => row.is_cool).length

Answer 1

You can do this using jus a query .. try using a sum on boolend and group by
select date(created), sum(is_cool)
from my_table
group by date(created)
or if you need the number of users
select t.date_created, count(*) num_user
from (
select distinct date(created) date_created, user_id
from my_table
where is_cool = TRUE
) t
group by t.date_created
or if need the last value for is_cool
select date(max_date), sum(is_cool)
from (
select t.user_id, t.max_date, m.is_cool, m.user_id
from my_table m
inner join (
select max(date_created) max_date, user_id
from my_table
group by user_id, date(date_created)
) t on t.max_date = m.date_created
and t.user_id = m.user_id
where m.is_cool = TRUE
) t2
group by date(max_date)

Answer 2

A correlated subquery might be the simplest solution. The following gets the value of is_cool for each user on each date:
select u.user_id, d.date,
(select t.is_cool
from t
where t.user_id = u.user_id and
t.created < dateadd(day, 1, d.date)
order by t.created desc
limit 1
) as is_cool
from (select distinct date(created) as date
from t
) d cross join
(select distinct user_id
from t
) u ;
Then aggregate:
select date, sum(is_cool)
from (select u.user_id, d.date,
(select t.is_cool
from t
where t.user_id = u.user_id and
t.created < dateadd(day, 1, d.date)
order by t.created desc
limit 1
) as is_cool
from (select distinct date(created) as date
from t
) d cross join
(select distinct user_id
from t
) u
) ud
group by date;