MySQL GROUP BY DIV result not consistent - mysql

Question

I'm trying to group data on 1 day interval using GROUP BY DIV as mentioned on this post:
Grouping into interval of 5 minutes within a time range
It looks fine on first glance.
But I notice inconsistency when comparing queries on 2 different date interval (but intersected).
First I use date range from Feb 01 00:00 to Feb 26 00:00,
second I use date range from Feb 20 00:00 to Feb 26 00:00
The values on Feb 20 are different between those 2 queries. But the rest (21 - 25) are matched.
Any idea what's going on & how to fix it?
Update:
Here's the stored procedure to generate dummy data on February on each minute:
DELIMITER $$
CREATE DEFINER=`root`#`127.0.0.1` PROCEDURE `testdata`()
BEGIN
DECLARE gap int;
DECLARE x bigint;
SET gap = 60000;
SET x = 1454265000000;
CREATE TABLE IF NOT EXISTS testdata (
timestamp bigint(20) default NULL,
value int(20) default NULL
)
ENGINE=MyISAM DEFAULT CHARSET=utf8;
WHILE x <= 1456770599000 DO
INSERT INTO testdata(timestamp, value) VALUES (x, FLOOR(RAND() * (270 + 1)) + 30);
SET x = x + gap;
END WHILE;
select x;
END
And here're 2 queries to compare 2 interval:
select from_unixtime(timestamp / 1000), count(value) from testdata where timestamp >= 1454265000000 and timestamp <= 1456770599000 group by timestamp div 86400000;
select from_unixtime(timestamp / 1000), count(value) from testdata where timestamp >= 1455906600000 and timestamp <= 1456770599000 group by timestamp div 86400000;
First query at 2016-02-20 return 1440. Second query at 2016-02-20 return 2 records at 2016-02-20 00:00:00 = 330 and at 2016-02-20 05:30:00 = 1440.

Answer 1

The duplication is because your server's timezone isn't the same as UTC. Unix timestamps are based on the time in UTC, so timestamp DIV 86400000 is grouping by UTC dates. But FROM_UNIXTIME() will return a time in the database's timezone. Since you're selecting FROM_UNIXTIME(timestamp/1000), you're selecting an arbitrary row within the group, and the date of that in the server's timezone may be different from its UTC date. As a result, two different UTC date groups will show the same timestamp date.
What you should do is select the date in UTC, so you're displaying the same date that you're grouping by.
SELECT FROM_UNIXTIME((TIMESTAMP DIV 86400000) * 86400), COUNT(*)
FROM testdata
WHERE timestamp BETWEEN 1455906600000 and 1456770599000
GROUP BY TIMESTAMP DIV 86400000