MySQL 8.0

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CREATE TABLE urls
    (`id` int, `url` varchar(10))
INSERT INTO urls
    (`id`, `url`)
VALUES
    (1, 'google.com'),
    (2, 'apple.com'),
    (3, 'bing.com')
CREATE TABLE ratings
    (`id` int, `rating` int, `url_id` int, `created_at` varchar(10))
INSERT INTO ratings
    (`id`, `rating`, `url_id`, `created_at`)
VALUES
    (1, 10, 1, '2018-10-10'),
    (2, 5, 1, '2018-10-12'),
    (3, 8, 2, '2018-10-10'),
    (4, 11, 1, '2018-11-10'),
    (5, 5, 2, '2018-11-10'),
    (6, 8, 2, '2018-11-15'),
    (7, 6, 3, '2018-11-25')
SELECT u.url, 
       AVG(CASE WHEN MONTH(r.created_at) = MONTH(CURDATE() - INTERVAL 1 MONTH)
                 AND YEAR(r.created_at)  = YEAR(CURDATE() - INTERVAL 1 MONTH) THEN rating END) AS `previous rating`,
       AVG(CASE WHEN MONTH(r.created_at) = MONTH(CURDATE())
                 AND YEAR(r.created_at) = YEAR(CURDATE())THEN rating END) AS `current rating`
FROM urls u
LEFT JOIN ratings r
ON r.url_id = u.id
GROUP BY u.url
ORDER BY `current rating` DESC


url
previous rating
current rating


google.com
7.5000
11.0000

apple.com
8.0000
6.5000

bing.com
null
6.0000


SELECT u.url,
       pr.rating AS `previous rating`,
       cr.rating AS `current rating`
FROM urls u
LEFT JOIN (SELECT url_id, rating
           FROM ratings r1
           WHERE created_at = (SELECT MAX(created_at)
                               FROM ratings r2
                               WHERE r2.url_id = r1.url_id AND
                                   MONTH(created_at) = MONTH(CURDATE() - INTERVAL 1 MONTH) AND
                                   YEAR(created_at)  = YEAR(CURDATE() - INTERVAL 1 MONTH)
                               )
           ) pr ON pr.url_id = u.id
LEFT JOIN (SELECT url_id, rating
           FROM ratings r1
           WHERE created_at = (SELECT MAX(created_at)
                               FROM ratings r2
                               WHERE r2.url_id = r1.url_id
                               )
           ) cr ON cr.url_id = u.id
ORDER BY `current rating` DESC


url
previous rating
current rating


google.com
5
11

apple.com
8
8

bing.com
null
6
url	previous rating	current rating
google.com	7.5000	11.0000
apple.com	8.0000	6.5000
bing.com	null	6.0000