Group by data to get count between the datetime range - sql

Question

Let's say I've a table like below
start_time end_time user_name
2019-01-01 00:00:05 2019-01-01 00:05:05 user1
2019-01-01 00:01:35 2019-01-01 00:06:05 user2
2019-01-01 00:02:05 2019-01-01 00:07:05 user3
2019-01-01 00:03:05 2019-01-01 00:08:05 user1
2019-01-01 00:04:05 2019-01-01 00:09:05 user2
My objective is find out how many users were logged in for a MINUTE. Say like below
time active no of users
2019-01-01 00:00:00 1
2019-01-01 00:01:00 2
2019-01-01 00:02:00 3
2019-01-01 00:03:00 3
2019-01-01 00:04:00 3
Now I first tried to round of time for a new column dateadd(mi, datediff(mi, 0, dateadd(s, 30, start_time)), 0). So, I will receive like above table time column
Next I tried to find the count for rounded datetime like below
SELECT
dateadd(mi, datediff(mi, 0, dateadd(s, 30, start_time)), 0) as RoundedDateTime,
(
SELECT count(distinct(user_name))
FROM entrytable sh
WHERE (sh.end_time > dateadd(mi, datediff(mi, 0, dateadd(s, 30, t.start_time)), 0)
and sh.start_time <= dateadd(mi, datediff(mi, 0, dateadd(s, 30, t.start_time)), 0))
) as usercounter
FROM entrytable t
But, above SQL query is running for longer time and goes to not responding mode.
I could not fix the issue. Can someone help?
Thanks in advance!

Answer 1

The most trivial solution is this:
DECLARE #t TABLE (start_time datetime, end_time datetime, user_name varchar(10));
INSERT INTO #t VALUES
('2019-01-01 00:00:05', '2019-01-01 00:05:05', 'user1'),
('2019-01-01 00:01:35', '2019-01-01 00:06:05', 'user2'),
('2019-01-01 00:02:05', '2019-01-01 00:07:05', 'user3'),
('2019-01-01 00:03:05', '2019-01-01 00:08:05', 'user1'),
('2019-01-01 00:04:05', '2019-01-01 00:09:05', 'user2');
SELECT dt AS date_time, SUM(SUM(val)) OVER (ORDER BY dt) AS active_count
FROM (
SELECT start_time, +1 FROM #t UNION ALL
SELECT end_time, -1 FROM #t
) cte1(dt, val)
GROUP BY dt
This will give you the number of active users whenever there was a change (someone logged in or logged out). Result:
| date_time | active_count |
|-------------------------|--------------|
| 2019-01-01 00:00:05.000 | 1 |
| 2019-01-01 00:01:35.000 | 2 |
| 2019-01-01 00:02:05.000 | 3 |
| 2019-01-01 00:03:05.000 | 4 |
| 2019-01-01 00:04:05.000 | 5 |
| 2019-01-01 00:05:05.000 | 4 |
| 2019-01-01 00:06:05.000 | 3 |
| 2019-01-01 00:07:05.000 | 2 |
| 2019-01-01 00:08:05.000 | 1 |
| 2019-01-01 00:09:05.000 | 0 |
Be advised that the result does not contain the "in-between" dates.

Answer 2

This question was originally tagged for SQL Server 2012, so this answer is for SQL Server.
One method is to generate a list of minutes and then:
with minutes as (
select cast('2019-01-01 00:00:00' as datetime) as mm
union all
select dateadd(minute, 1, minute)
from cte
where mm < '2019-01-01 00:00:05'
)
select m.*,
(select count(*)
from entrytable et
where et.start_time <= m.mm and
et.end_time > m.mm
) as num_actives
from minutes m;