Database Error

Expression #3 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'siglotechblog.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(posts.created_at) AS year,
       MONTH(posts.created_at) AS month,
       posts.created_at AS created_at
FROM posts
WHERE (posts.status = 'public')
GROUP BY YEAR(posts.created_at),
         MONTH(posts.created_at)
ORDER BY posts.created_at DESC


Array
(
    [:status] => public
)
#0 /var/www/siglotech/blog/includes/class/SQL.php(220): Query->__construct(Object(SQL), 'SELECT DISTINCT...', Array, false)
#1 /var/www/siglotech/blog/includes/class/SQL.php(257): SQL->query('SELECT DISTINCT...', Array, false)
#2 /var/www/siglotech/blog/includes/controller/Main.php(295): SQL->select('posts', Array, Array, Array, Array, NULL, NULL, Array)
#3 [internal function]: MainController->archive()
#4 /var/www/siglotech/blog/includes/class/Route.php(115): call_user_func_array(Array, Array)
#5 /var/www/siglotech/blog/index.php(22): Route->init()
#6 {main}