MYSQL max() and group by error:only_full_group_by - mysql

Question

I have question about a MySQL query that is logging error's since updating the MySQL-5.7.
The error is the "only_full_group_by" which is will spoken off on stackoverflow.
In many answers it's stated not to disable this option but improve your sql query.
The query that I'm using is returning the minimum and maximum values of a counter per hour.
SELECT MAX( counter ) AS max,
MIN( counter ) AS min,
DATE_FORMAT(date_time, '%H:%i') AS dt
FROM table1
WHERE date_time >= NOW() - INTERVAL 1 DAY
GROUP BY YEAR(date_time), MONTH(date_time), DAY(date_time), HOUR(date_time)
as I understand from the error message I'm missing one of the items from the SELECT cause in the GROUP BY cause. But however I restort/remove/add items I'm not getting the result I got before the upgrade to MySQL-5.7.
I tried to subquery the main query to improve the SQL query. But somehow I can't recreate the results.
What is it I'm missing?

Answer 1

MySQL isn't able to determine the functional dependence ... between the expressions in the GROUP BY clause, and the expressions in the SELECT list.
The non-aggregate expression in the SELECT list (DATE_FORMAT(date_time, '%H:%i') includes a minutes component. The GROUP BY clause is going to collapse the rows into groups by just hour. So the value of the minutes is indeterminate... we know it's going to come from some row in the group, but there's no guarantee which one.
(The question reference to ONLY_FULL_GROUP_BY seems to indicate that we've got some understanding of indeterminate values...)
The easiest (fewest) changes fix would be to wrap that expression in a MIN or MAX function.
SELECT MAX(t.counter) AS `max`
, MIN(t.counter) AS `min`
, MIN(DATE_FORMAT(t.date_time,'%H:%i')) AS `dt`
FROM table1 t
WHERE t.date_time >= NOW() - INTERVAL 1 DAY
GROUP
BY YEAR(t.date_time)
, MONTH(t.date_time)
, DAY(t.date_time)
, HOUR(t.date_time)
ORDER
BY YEAR(t.date_time)
, MONTH(t.date_time)
, DAY(t.date_time)
, HOUR(t.date_time)
If we want rows returned in a particular order, we should include an ORDER BY clause, and not rely on MySQL-specific extension or behavior of GROUP BY (which may disappear in future releases.)
It's a bit odd to be doing a GROUP BY year, month, day and not including those values in the SELECT list. (It's not invalid to do that, just kind of strange. The conditions in the WHERE clause are guaranteeing that we don't have more than 24 hours span for date_time.
My preference would to do the GROUP BY on the same expression as the non-aggregate in the SELECT list. If I ever needed more than 24 hours, I'd include the date component:
SELECT MAX(t.counter) AS `max`
, MIN(t.counter) AS `min`
, DATE_FORMAT(t.date_time,'%Y-%m-%d %H:00') + INTERVAL 0 DAY AS `dt`
FROM table1 t
WHERE t.date_time >= NOW() - INTERVAL 1 DAY
GROUP
BY DATE_FORMAT(t.date_time,'%Y-%m-%d %H:00') + INTERVAL 0 DAY
ORDER
BY DATE_FORMAT(t.date_time,'%Y-%m-%d %H:00') + INTERVAL 0 DAY
--or--
if we always know it's just one day's worth of date_time, and we only want to return the hour, then we can group by just the hour. The same expression as in the SELECT list.
SELECT MAX(t.counter) AS `max`
, MIN(t.counter) AS `min`
, DATE_FORMAT(t.date_time,'%H:00') AS `dt`
FROM table1 t
WHERE t.date_time >= NOW() - INTERVAL 1 DAY
GROUP
BY DATE_FORMAT(t.date_time,'%H:00')
, DATE_FORMAT(t.date_time,'%Y-%m-%d %H')
ORDER
BY DATE_FORMAT(t.date_time,'%Y-%m-%d %H')

Answer 2

SELECT MAX( counter ) AS max,
MIN( counter ) AS min,
YEAR(date_time) AS g_year,
MONTH(date_time)AS g_month,
DAY(date_time) AS g_day,
HOUR(date_time) AS g_hour
FROM table1
WHERE date_time >= NOW() - INTERVAL 1 DAY
GROUP BY g_year, g_month, g_day, g_hour
Or you can get rid of redundant data if you always do it for 1 day:
SELECT MAX( counter ) AS max,
MIN( counter ) AS min,
DAY(date_time) AS g_day,
HOUR(date_time) AS g_hour
FROM table1
WHERE date_time >= NOW() - INTERVAL 1 DAY
GROUP BY g_day, g_hour