Database Query to generate a Time-based Chart - sql

Question

I have a logins table in the following (simplified) structure:
id | login_time
---------
1 | 2019-02-04 18:14:30.026361+00
2 | 2019-02-04 22:10:19.720065+00
3 | 2019-02-06 15:51:53.799014+00
Now I want to generate chart like this:
https://prnt.sc/mifz6y
Basically I want to show the logins within the past 48 hours.
My current query:
SELECT count(*), date_trunc('hour', login_time) as time_trunced FROM user_logins
WHERE login_time > now() - interval '48' hour
GROUP BY time_trunced
ORDER BY time_trunced DESC
This works as long as there are entries for every hour. However, if in some hour there were no logins, there will be no entry selected, like this:
time_trunced | count
---------------------
12:00 | 1
13:00 | 2
15:00 | 3
16:00 | 5
I would need a continous query, so that I can simply put the count values into an array:
time_trunced | count
---------------------
12:00 | 1
13:00 | 2
14:00 | 0 <-- This is missing
15:00 | 3
16:00 | 5
Based on that I can simply transform the query result into an array like [1, 2, 0, 3, 5] and pass that to my frontend.
Is this possible with postgresql? Or do I need to implement my own logic?

Answer 1

I think I would do:
select gs.h, count(ul.login_time)
from generate_series(
date_trunc('hour', now() - interval '48 hour'),
date_trunc('hour', now()),
interval '1 hour'
) gs(h) left join
user_logins ul
on ul.login_time >= gs.h and
ul.login_time < gs.h + interval '1 hour'
group by gs.h
order by gs.h;

Answer 2

This can almost certainly be tidied up a bit but should give you some ides. Props to clamp for the generate_series() tip :
SELECT t.time_trunced,coalesce(l.login_count,0) as logins
FROM
(
-- Generate an inline view with all hours between the min & max values in user_logins table
SELECT date_trunc('hour',a.min_time)+ interval '1h' * b.hr_offset as time_trunced
FROM (select min(login_time) as min_time from user_logins) a
JOIN (select generate_series(0,(select ceil((EXTRACT(EPOCH FROM max(login_time))-EXTRACT(EPOCH FROM min(login_time)))/3600) from user_logins)::int) as hr_offset) b on true
) t
LEFT JOIN
(
-- OP's original query tweaked a bit
SELECT count(*) as login_count, date_trunc('hour', login_time) as time_trunced
FROM user_logins
GROUP BY time_trunced
) l on t.time_trunced=l.time_trunced
order BY 1 desc;