mysql avg value with specified number of rows - mysql

Question

i have a simple table with
value (int), created (timestamp)
i would like to do a query that return an arbitrary number of rows with avg (value) and avg(created). The grouping function is the order by of created, means that if i ask for 2 rows, i should obtain a set based on the first rows ordered by created.
i have the following table:
create table log(value int,created timestamp);
insert into log values
(1,'2016-01-01 00:00:00'),
(2,'2016-01-01 01:00:00'),
(3,'2016-01-01 02:00:00'),
(4,'2016-01-01 03:00:00'),
(5,'2016-01-01 04:00:00'),
(6,'2016-01-01 05:00:00'),
(7,'2016-01-01 06:00:00'),
(8,'2016-01-01 07:00:00'),
(9,'2016-01-01 08:00:00');
http://sqlfiddle.com/#!9/b9a94
i want to retrive 3 rows that should be
2-2016-01-01 01:00:00
5-2016-01-01 04:00:00
8-2016-01-01 07:00:00
it's possible to do it in a single query without using java or php processing?

Answer 1

For example when you group by 3, ordered by creation date. You can change 3 to the size you want to group by.
SELECT avg(sub.value), avg(sub.created)
from (
SELECT #row_number:=#row_number+1 AS row_number,value, created
FROM log, (SELECT #row_number:=0) AS t
ORDER BY created
) sub
group by floor((sub.row_number-1)/3)
UPDATE
if you always want it to be divided in 3 groups based on the number of rows you can do the following:
SELECT avg(sub.value), avg(sub.created)
from (
SELECT #row_number:=#row_number+1 AS row_number, floor((#row_number-1)/c.num) groupid,value, created
FROM log,
(SELECT #row_number:=0) AS t,
(SELECT ceil(COUNT(*)/3) num FROM log) c
ORDER BY log.created
) sub
GROUP BY sub.groupid