请问 MySQL 的这两个语句有啥区别呢？ [ 2.0 版本 ]

select count(*) as count,DATE_FORMAT(add_time,'%Y年%m月') as date from access_log  GROUP BY DATE_FORMAT(add_time,'%Y-%m');

select count(*) as count,DATE_FORMAT(add_time,'%Y年%m月') as date from access_log  GROUP BY (SELECT DATE_FORMAT(add_time,'%Y-%m'));

请问这两个语句有啥区别呢？