AVG limited to 3 last values for each group - mysql

Question

Basically i have two tables:
Here's code to create two tables if this can help someone who will be willing to help me:
CREATE TABLE IF NOT EXISTS `coefficients` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`datetime` datetime NOT NULL,
`campaign_id` int(11) NOT NULL,
`score` decimal(10,2) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=9 DEFAULT CHARSET=utf8;
INSERT INTO `coefficients` (`id`, `datetime`, `campaign_id`, `score`) VALUES
(1, '2017-01-29 22:32:13', 1, 20.00),
(2, '2017-01-29 22:36:22', 1, 34.00),
(3, '2017-01-29 22:36:30', 1, 30.00),
(4, '2017-01-29 22:36:43', 1, 1000.00),
(5, '2017-01-29 22:37:13', 2, 10.00),
(6, '2017-01-29 22:37:26', 2, 15.00),
(7, '2017-01-29 22:37:43', 2, 20.00),
(8, '2017-01-29 22:30:51', 2, 1000.00);
CREATE TABLE IF NOT EXISTS `statistics` (
`id` int(11) NOT NULL AUTO_INCREMENT,
`campaign_id` int(11) NOT NULL,
`stats1` int(11) NOT NULL,
`stats2` int(11) NOT NULL,
PRIMARY KEY (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=3 DEFAULT CHARSET=utf8;
INSERT INTO `statistics` (`id`, `campaign_id`, `stats1`, `stats2`) VALUES
(1, 1, 34, 38),
(2, 2, 23, 45);
I would like to get average coefficient for each campaign_id calculated based on latest 3 logged coefficients for each campaign_id.
Here's screenshot of two tables and result that i need to get:
data + result (visual representation)
The main problem is that i have no idea how to join these two tables if i need only average coefficient for each campaign_id based on 3 latest logged nu,bers for it :(
I will appreciate any help

Answer 1

Following query will give you the top 3 records per campaign_id from coefficients table:
SET #currcount = NULL, #currvalue = NULL;
SELECT id, campaign_id, score, c_index FROM (
SELECT
id, campaign_id, score,
#currcount := IF(#currvalue = campaign_id, #currcount + 1, 1) AS c_index,
#currvalue := campaign_id AS s
FROM coefficients
order by id
) AS a where c_index <= 3
Now, all you have to do is, add a GROUP BY to this query, calculate average score and join it with statistics table, e.g.:
SET #currcount = NULL, #currvalue = NULL;
SELECT a.id, a.campaign_id, avg(score), c_index, s.stats1, s.stats2 FROM (
SELECT
id, campaign_id, score,
#currcount := IF(#currvalue = campaign_id, #currcount + 1, 1) AS c_index,
#currvalue := campaign_id AS s
FROM coefficients
order by id
) AS a join statistics s on a.campaign_id = s.campaign_id
where c_index <= 3
group by campaign_id
Here's the SQL Fiddle.

Answer 2

In MySQL, the best way is usually to use variables. Getting the statistics is just a join, so that is not interesting. Let's get the average from the coefficients table:
select c.campaign_id, avg(c.score) as avg_score
from (select c.*,
(#rn := if(#c = c.campaign_id, #rn + 1,
if(#c := c.campaign_id, 1, 1)
)
) as rn
from coefficients c cross join
(select #rn := 0, #c := -1) params
order by c.campaign_id, c.datetime desc
) c
where rn <= 3
group by c.campaign_id;