日付列をYYYYMMDDHH24MISS形式で出力
select date_format(created_at, '%Y%m%d%H%i%s')
from TABLE;
日単位で件数をサマリーする
select date_format(created_at, '%Y%m%d'),
count(*)
from TABLE
group by date_format(created_at, '%Y%m%d') desc
order by date_format(created_at, '%Y%m%d') desc;
1時間単位で件数をサマリーする
select date_format(created_at, '%Y%m%d_%H'),
count(*)
from TABLE
group by date_format(created_at, '%Y%m%d_%H') desc
order by date_format(created_at, '%Y%m%d_%H') desc;
こちらもおススメ