Database Error

Expression #3 of SELECT list is not in GROUP BY clause and contains nonaggregated column 'jamestow_chyr759.chy_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(chy_posts.created_at) AS year,
       MONTH(chy_posts.created_at) AS month,
       chy_posts.created_at AS created_at
FROM chy_posts
WHERE (chy_posts.status = 'public')
GROUP BY YEAR(chy_posts.created_at),
ORDER BY chy_posts.created_at DESC

    [:status] => public
#0 /home/jamestow/public_html/ Query->__construct(Object(SQL), 'SELECT DISTINCT...', Array, false)
#1 /home/jamestow/public_html/ SQL->query('SELECT DISTINCT...', Array, false)
#2 /home/jamestow/public_html/ SQL->select('posts', Array, Array, Array, Array, NULL, NULL, Array)
#3 [internal function]: MainController->archive()
#4 /home/jamestow/public_html/ call_user_func_array(Array, Array)
#5 /home/jamestow/public_html/ Route->init()
#6 {main}