MySQL find number of students in attendance broken down by time - mysql

Question

I have a table containing arriving and departing times for students attending a class. Given something like this data:
CREATE TABLE `attendance` (
`id` int(11) unsigned NOT NULL AUTO_INCREMENT,
`class_id` int(11) DEFAULT NULL,
`student_id` int(11) NOT NULL DEFAULT '0',
`arrival` datetime DEFAULT NULL,
`departure` datetime DEFAULT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO `attendance` (`id`, `class_id`, `student_id`, `arrival`, `departure`)
VALUES
(1,1,1,'2013-01-01 16:00:00','2013-01-01 17:00:00'),
(2,1,2,'2013-01-01 16:00:00','2013-01-01 18:00:00'),
(3,1,3,'2013-01-01 17:00:00','2013-01-01 19:00:00'),
(4,1,4,'2013-01-01 17:00:00','2013-01-01 19:00:00'),
(5,1,5,'2013-01-01 17:30:00','2013-01-01 18:30:00');
I'm trying to get a breakdown of time in minutes, and how many students are present for that time period. A result something like this from the above data:
Time Students
60 2 (the first hour from 16:00 to 17:00 has students 1 & 2)
30 3 (the next 30 minutes from 17:00 to 17:30 has students 2, 3 & 4)
30 4 (etc...)
30 3
30 2
The select statement I have so far is getting some way towards the answer but I can't quite get it working:
SELECT a.id, a.arrival, b.id, LEAST(a.departure,b.departure) AS departure,
TIMEDIFF((LEAST(a.departure,b.departure)),(a.arrival)) AS subtime
FROM attendance a
JOIN attendance b ON (a.id <> b.id and a.class_id=b.class_id
and a.arrival >= b.arrival and a.arrival < b.departure)
WHERE a.class_id=1
ORDER BY a.arrival, departure, b.id;
Thank you in advance to anyone who can help me get this right.

Answer 1

Using correlated sub-queries you can create virtual tables (not the same as a temporary table, but kinda the same idea). You can then query against these virtual tables just as if they really existed.
select clocks.clock, count( att.student_id ) as numStudents
from
(
( select arrival as clock from attendance )
union distinct
( select departure as clock from attendance )
)
as clocks
left outer join attendance att on att.arrival <= clocks.clock and clocks.clock < att.departure
group by clocks.clock
order by 1,2
;
Almost what you are looking for. Rather than group by elapsed time, this uses the actual 'event' timestamps (arrivals and departures) and gives you a useful report.
clock numStudents
------------------- -----------
2013-01-01 16:00:00 2
2013-01-01 17:00:00 3
2013-01-01 17:30:00 4
2013-01-01 18:00:00 3
2013-01-01 18:30:00 2
2013-01-01 19:00:00 0
The report shows how many students are still 'here' at each event time.
Hopefully this is useful for you.