Group datetime within range - mysql

Question

I have some troubles in grouping some datasets. Here is my query and query result.
SELECT COUNT(*),salarie_id, created_at
FROM enregistrement
GROUP BY salarie_id, created_at
My point is to group similar created_at rows, in an interval of +/- 3 sec. I didn't manage it, even in using INTERVAL, in such criterias as HAVING
, WHERE ... BETWEEN.
How can I group these rows to get, for example 36 (33+3) in my count result, as shown in the image ?
Didn't found any suitable solution... Let me know if you want some additional information.
UPDATE 1 : Looks like #fancypants solution is on the right way.
SELECT COUNT(*),salarie_id, DATE_FORMAT(created_at, CONCAT('%Y-%m-%d %H:%i:', LEFT(RIGHT(created_at, 2), 1), RIGHT(created_at, 1) % 3)) AS 'date'
FROM enregistrement
GROUP BY salarie_id, DATE_FORMAT(created_at, CONCAT('%Y-%m-%d %H:%i:', LEFT(RIGHT(created_at, 2), 1), RIGHT(created_at, 1) % 3))

Answer 1

SELECT COUNT(*),salarie_id, created_at
FROM enregistrement
GROUP BY salarie_id,
DATE_FORMAT(created_at, CONCAT('%Y-%m-%d %H:%i:', LEFT(RIGHT(created_at, 2), 1),
CASE WHEN RIGHT(created_at, 1) BETWEEN 1 AND 4 THEN 0
WHEN RIGHT(created_at, 1) BETWEEN 5 AND 7 THEN 1
ELSE 2 END
));
You can use DATE_FORMAT() to bring your datetime/timestamp column in certain shapes. In your case we specify the format with
CONCAT('%Y-%m-%d %H:%i:', LEFT(RIGHT(created_at, 2), 1), RIGHT(created_at, 1) % 3))
That is, we take the year, the month, the day, the hour, the minute. Then we concatenate the date format string with the power of ten of seconds.
For example you have this seconds:
21
33
36
40
Then we take 2, 3, 3 and 4 with LEFT(RIGHT(created_at, 2), 1). The 1, 3, 6 and 0 we get with RIGHT(created_at, 1). Then you just have to put them into self-defined groups with CASE WHEN.

Answer 2

You need to create the column in your select, and then you can group by this column. Here is an example:
SELECT
COUNT(*),
salarie_id,
IF(created_at BETWEEN "2014-10-01 00:00:00" AND "2014-10-01 00:00:00", "2014-10-01 00:00:00"
ELSEIF(created_at BETWEEN "2015-02-18 11:05:43" AND "2015-02-18 11:05:46", "2015-02-18 11:05:43",
ELSEIF( ...
))) as buckets
FROM enregistrement
GROUP BY salarie_id, bucket
I am not sure how practical this may be in your case though, since you probably do not know the intervals beforehand.
An easy way to get through in your example would be to simply group by date and time without the seconds:
SELECT
DATE_FORMAT(created_at,"%Y-%m-%d %H:%i") as short_date
.....
GROUP BY salarie_id, short_date