Find number of rows during a commun interval - sql

Question

Let's say I have an employees list and I log inside "log" table the beginning and the end of the employee working day. Example:
ID
NAME
BEGINTIME
ENDTIME
1
Mary
05/04/2021 07:10
05/04/2021 10:10
2
John
05/04/2021 09:10
05/04/2021 14:10
And I want to search: how many employees where working at the same time (interval) and the number of them (if more than 1) and sort them from max to min.
Example: Mary and John were working at the same time May 4th from 09:10 to 10:10, before 09:10 Mary was alone, then John came at 10:10 and they were two employee working at the office (the max in that example) then Mary left the office at 10:10 and John was alone.
The point is due to COVID19, I want to know for on a specific date (May 4th in that example) when we had the more employees working in the office at the same time, to split them to lower that max.
Exepected result (the first row is to define the aim) :
MAXEMPLOYEES
FROMBEGINTIME
TOENDTIME
LISTAGG
120
dd/MM/YYYY HH:mm
dd/MM/YYYY HH:mm
...
2
05/04/2021 09:10
05/04/2021 10:10
Mary, John
I really don't know where to start, maybe I thought, find all the log for the May 4th (let's say 1000 row) and loop, first row then loop again to the other logs and if MYSELECTEDROWBEGINTIME >= NthBEGINTIME AND MYSELECTEDROWENDTIME <= NthENDTIME and loop to my list and then go the 2nd row etc.

Answer 1

For 12-12:30 interval, you can start working on this query:
select count(*) employees_count
from log
where begintime<to_date('05/04/2021 12:30','MM/DD/YYYY HH24:MI')
and endtime>to_date('05/04/2021 12:00','MM/DD/YYYY HH24:MI');
I'm assuming that the begintime and endtime are of date type. In general, your condition should be begintime<interval_endtime and endtime>interval_begintime.
For your example output in your original post, you may need a table for the time interval to be joined to log, which would look like this:
int_id
int_from
int_to
1
09:10
10:10
2
12:00
12:30
and your query for May 4, 2021 can be something like this:
select count(*) maxemployee,
int_from frombegintime,
int_to toendtime
from (
select log.id, log.begintime, log.endtime,
tint.int_from, tint.int_to
from log
join time_interval tint
on log.begintime<to_date('05/04/2021 '||tint.int_to,'MM/DD/YYYY HH24:MI')
and log.endtime>to_date('05/04/2021 '||tint.int_from,'MM/DD/YYYY HH24:MI')
) t
group by int_from, int_to