Mysql: how to find users that currently in the room? - mysql

Question

I have table entrances that logs times when users came into the room and came out of the room. Something like that:
user | action | time
-------------------------------------------
Ivan | in | 2016-08-28 12:00:00
John | in | 2016-08-28 12:00:01
Ann | in | 2016-08-28 12:00:02
Ivan | out | 2016-08-28 12:00:03
Ivan | in | 2016-08-28 12:00:04
Ann | out | 2016-08-28 12:00:05
Ivan | out | 2016-08-28 12:00:06
Mike | in | 2016-08-28 12:00:07
John | out | 2016-08-28 12:00:08
Ann | out | 2016-08-18 12:00:09
John | in | 2016-08-18 12:00:10
John | out | 2016-08-18 12:00:11
Ann | in | 2016-08-18 12:00:12
Users actions are independent. Only is known is that first action is always in and user cannot in twice witout out (and reverse).
My goal is to find all users that currently in room.
I have two ideas:
select users that have no out after latest in
select users that has count in more that count out
How to implement this on mysql? Or any other ideas?
SQL for testing:
CREATE TABLE `entrances` (
`id` int(11) NOT NULL,
`user` varchar(10) COLLATE utf8_bin NOT NULL,
`action` varchar(3) COLLATE utf8_bin NOT NULL,
`time` datetime NOT NULL
) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_bin;
INSERT INTO `entrances` (`id`, `user`, `action`, `time`) VALUES
(1, 'Ivan', 'in', '2016-08-28 12:00:00'),
(2, 'John', 'in', '2016-08-28 12:00:01'),
(3, 'Ann', 'in', '2016-08-28 12:00:02'),
(4, 'Ivan', 'out', '2016-08-28 12:00:03'),
(5, 'Ivan', 'in', '2016-08-28 12:00:04'),
(6, 'Ann', 'out', '2016-08-28 12:00:05'),
(7, 'Ivan', 'out', '2016-08-28 12:00:06'),
(8, 'Mike', 'in', '2016-08-28 12:00:07'),
(9, 'John', 'out', '2016-08-28 12:00:08'),
(10, 'Ann', 'out', '2016-08-28 12:00:09'),
(11, 'John', 'in', '2016-08-28 12:00:10'),
(12, 'John', 'out', '2016-08-28 12:00:11'),
(13, 'Ann', 'in', '2016-08-28 12:00:12');
ALTER TABLE `entrances` ADD PRIMARY KEY (`id`);

Answer 1

First get the last action time from each user in the subquery
Then join against that subquery to have only the last record for each user
Then take only those records having the action = in with the where clause
Like this
select e.*
from entrances e
join
(
select user, max(time) as mtime
from entrances
group by user
) t on t.user = e.user
and t.mtime = e.time
where e.action = 'in'

Answer 2

Join all row with the action out and select the ones that have no out action or the in action is newer
SELECT * FROM entrances e
LEFT JOIN entrances e1 ON e.user = e1.user AND e1.action = 'out'
WHERE e.action = 'in' AND (e1.user IS NULL OR e.time > e1.time)
GROUP BY e.user, e.action

Answer 3

I will try something like this however it's better to test on real data.
select user from entrances e inner join
(select user, count(*) cnt from entrances where action='in') as e_in
on e.user=e_in.user left join
(select user, count(*) cnt from entrances where action='out') as e_out
on e.user=e_out.user
where e_out.user is null or (e_in.cnt - e_out.cnt) = 1

Answer 4

Mysql implementation of my idea number 1:
SELECT e.* FROM entrances e
WHERE NOT EXISTS (
SELECT * FROM entrances WHERE e.user=user AND action="out" AND time > (
SELECT MAX(time) FROM entrances WHERE e.user=user AND action="in"
)
)
This works literally as written: find users that have no out after last in. Utilizes NOT EXISTS feature. Advantage is readability without magic, plain algorythm.