Database Error

Expression #3 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'joek_chyr349.chyxy_posts.created_at' which is not functionally dependent on columns in GROUP BY clause; this is incompatible with sql_mode=only_full_group_by
SELECT DISTINCT YEAR(chyxy_posts.created_at) AS year,
       MONTH(chyxy_posts.created_at) AS month,
       chyxy_posts.created_at AS created_at
FROM chyxy_posts
WHERE (chyxy_posts.status = 'public')
GROUP BY YEAR(chyxy_posts.created_at),
         MONTH(chyxy_posts.created_at)
ORDER BY chyxy_posts.created_at DESC


Array
(
    [:status] => public
)
#0 /home/joek/public_html/includes/class/SQL.php(220): Query->__construct(Object(SQL), 'SELECT DISTINCT...', Array, false)
#1 /home/joek/public_html/includes/class/SQL.php(257): SQL->query('SELECT DISTINCT...', Array, false)
#2 /home/joek/public_html/includes/controller/Main.php(286): SQL->select('posts', Array, Array, Array, Array, NULL, NULL, Array)
#3 /home/joek/public_html/includes/class/Route.php(115): MainController->archive()
#4 /home/joek/public_html/index.php(22): Route->init()
#5 {main}