順番が変になったカラムを特定のキー順に振りなおしてやりなおせるSQL＠

ALTER TABLE `subject` CHANGE num num int(11) NOT NULL;
alter table subject drop primary key;
SET @i := 0; UPDATE subject SET num = (@i := @i +1) ORDER BY created;
ALTER TABLE subject CHANGE num num int auto_increment, ADD PRIMARY KEY(num);

ALTER TABLE subject AUTO_INCREMENT = @i;

実行は一瞬