Get data if insert are in the same second - php

Question

I have this table :
id idm date_play
1 5 2017-08-23 12:12:12
2 5 2017-08-23 12:12:12
3 6 2017-08-23 12:14:13
I want to identify if user has more then one insert in the same second. In the case describe I want to get the user id that is 5.
I tried like this :
SELECT `idm`, MAX(`s`) `conseq` FROM
(
SELECT
#s := IF(#u = `idm` AND (UNIX_TIMESTAMP(`date_play`) - #pt) BETWEEN 1 AND 100000, #s + 1, 0) s,
#u := `idm` `idm`,
#pt := UNIX_TIMESTAMP(`date_play`) pt
FROM table
WHERE date_play >= '2017-08-23 00:00:00'
AND date_play <= '2017-08-23 23:59:59'
ORDER BY `date_play`
) AS t
GROUP BY `idm`
Can you help me please ? Thx in advance and sorry for my english.

Answer 1

Assuming your dates are accurate down to the second level, you can do this with a single aggregation:
select idm
from t
group by idm
having count(*) > count(distinct date_play);
If date_play has fractional seconds, then you would need to remove those (say by converting to a string).
If you want the play dates where there are duplicates:
select idm, date_play
from t
group by idm, date_play
having count(*) >= 2;
Or, for just the idms, you could use select distinct with group by:
select distinct idm
from t
group by idm, date_play
having count(*) >= 2;
(I only mention this because this is the only type of problem that I know of where using select distinct with group by makes sense.)
If you want all the rows that are duplicated, I would go for exists instead:
select t.*
from t
where exists (select 1
from t t2
where t2.idm = t.idm and t2.date_play = t.date_play and
t2.id <> t.id
);
This should have reasonable performance with an index on (idm, date_play, id).

Answer 2

If your table is called mytable, the following should work:
SELECT t.`idm`
FROM mytable t INNER JOIN mytable t2
ON t.`idm`=t2.`idm` AND t.`date_play`=t2.`date_play` AND t.`id`!=t2.`id`
GROUP BY t.`idm`
Basically we join the table with itself, pairing records that have the same idm and date_play, but not the same id. This will have the effect of matching up any two records with the same user and datetime. We then group results by user so you don't get the same user id listed multiple times.
Edit:
Gordon Linoff and tadman's suggestions led me to this probably much more efficient query (credit to them)
SELECT t.`idm`
FROM mytable t
GROUP BY t.`date_play`
HAVING COUNT(t.`id`)>1