SELECT * FROM `article` where date_format(from_UNIXTIME(`add_time`),'%Y-%m-%d') = date_format(now(),'%Y-%m-%d');

 SELECT * FROM `article` where to_days(date_format(from_UNIXTIME(`add_time`),'%Y-%m-%d')) = to_days(now());

 SELECT * FROM `article` where to_days(`add_time`) = to_days(now());

 SELECT * FROM `article` where to_days(now()) – to_days(`add_time`) <= 1;

 SELECT * FROM `article` where date_sub(curdate(), INTERVAL 7 DAY) <= date(`add_time`);

 SELECT * FROM `article` where date_sub(curdate(), INTERVAL 30 DAY) <= date(`add_time`);

 SELECT * FROM `article` where date_format(`add_time`, ‘%Y%m') = date_format(curdate() , ‘%Y%m');

SELECT * FROM `article` where period_diff(date_format(now() , ‘%Y%m') , date_format(`add_time`, ‘%Y%m')) =1;

 
 mysql> select to_days('2010-11-22 14:39:51'); 
 +--------------------------------+ 
 | to_days('2010-11-22 14:39:51') |
 +--------------------------------+
 | 734463 |
 +--------------------------------+
 
 mysql> select to_days('2010-11-23 14:39:51');
 +--------------------------------+
 | to_days('2010-11-23 14:39:51') |
 +--------------------------------+
 | 734464 |
 +--------------------------------+

 mysql> select to_days('1997-10-07'), to_days('97-10-07');
 
 -> 729669, 729669

 
 mysql> select str_to_date("2010-11-23 14:39:51",'%Y-%m-%d %H:%i:%s');
 
 +--------------------------------------------------------+
 | str_to_date("2010-11-23 14:39:51",'%Y-%m-%d %H:%i:%s') |
 +--------------------------------------------------------+
 | 2010-11-23 14:39:51 |
 +--------------------------------------------------------+

select str_to_date(article.`add_time`,'%Y-%m-%d %H:%i:%s')
 from article
 where str_to_date(article.`add_time`,'%Y-%m-%d %H:%i:%s')>='2012-06-28 08:00:00' and str_to_date(article.`add_time`,'%Y-%m-%d %H:%i:%s')<='2012-06-28 09:59:59';