select date_format(created_at,'%Y-%m-%d') as `date`,count(*) from TABLE_NAME where date_format(created_at,'%Y-%m-%d') > '2014-01-01' group by `date` order by `date` asc
のようなクエリの場合
select `date`,count(*) from (select date_format(created_at,'%Y-%m-%d') as `date` from TABLE_NAME) s where `date` > '2014-01-01' group by `date` order by `date` asc
サブクエリを混ぜるしかありません
内容が薄いので、サブクエリ関連で参考になるURLでも貼っておきます^q^