比如需要执行的sql语句是：
SELECT f.msg_id, f.user_name, f.msg_title, f.msg_type, f.order_id, f.msg_status, f.msg_time, f.msg_area, COUNT(r.msg_id) AS reply FROM `ecs_feedback` AS f LEFT JOIN `ecs_feedback` AS r ON r.parent_id=f.msg_id WHERE f.parent_id = 0 GROUP BY f.msg_id ORDER by f.msg_id DESC
查询的数据库是别的数据库
如：M('feedback as a','ecs_','DB_CONFIG2');
