How do I count the max rows with the time range in SQL? - sql

Question

This is the table in Postgresql:
mydb=# \d login_log
Table "public.login_log"
Column | Type | Modifiers
-------------+--------------------------+-----------
id | integer |
login_start | timestamp with time zone |
login_end | timestamp with time zone |
some rows:
1 | 2015-03-19 10:00:00 | 2015-03-19 13:30:00
2 | 2015-03-19 10:20:00 | 2015-03-19 13:20:00
3 | 2015-03-19 13:00:00 | 2015-03-19 16:00:00
4 | 2015-03-19 13:10:00 | 2015-03-19 16:00:00
5 | 2015-03-19 14:30:00 | 2015-03-19 15:30:00
6 | 2015-03-19 15:00:00 | 2015-03-19 15:30:00
7 | 2015-03-19 12:00:00 | 2015-03-19 18:00:00
I need a SQL to count out in which time range there is the max logged users.
with the example above, the result is:
in time range: 2015-03-19 13:10:00 ~ 2015-03-19 13:20:00,
5 users logged in. (1, 2, 3, 4, 7)

Answer 1

Use range types (construct them "on the fly"). They offer quite a few helpful functions and operators. You would only need to define a custom aggregate, which will provide you the overall intersection. So - you would end up with something like this:
with common as (
select (intersection(tsrange(login_start, login_end))) as period
from login_log
)
select
-- common.period,
-- array_agg(id)
*
from common, login_log
WHERE tsrange(login_start, login_end) && common.period
-- GROUP BY common.period
/*
for some reason, when uncommenting the "^--..." lines,
and commenting the "*" one - sqlfiddle shows an empty result.
Nevertheless it works on my local posgres...
*/
See the working example: http://sqlfiddle.com/#!15/0c9c6/10

Answer 2

Find different timestamps of interest using UNION ALL, count number of active users at these timestamps:
select ts,
(select count(*) from login_log t2
where timestamps.ts between t2.login_start and t2.login_end) as count
from (select login_start as ts
from login_log
union all
select login_end
from login_log) as timestamps
order by count desc
fetch first 1 row only
Finally order descending and pick the highest value only!
(From a non Postgresql user, so some details may be wrong... Please comment if that's the case and I'll edit!)