MySQL日毎、周毎、月毎集計

１．日毎
　　group by date(created)

２．周毎
　・日曜日開始
　　group by date(created + interval 1 - DAYOFWEEK(created) day)
　・月曜日開始
　　group by date(created - interval (5 + DAYOFWEEK(created))%7 day)

３．月毎
　　group by date(created + interval 1 - DAYOFMONTH(created) day)
　　或いは
　　group by date_format(created, '%Y-%m')

※createdのデータ型はDATETIME