SELECT tpn.news_id, tpn.title, tpn.news_group_id, tpn.send_date FROM t_pm_news tpn WHERE 3 > ( SELECT COUNT(*) FROM t_pm_news t WHERE tpn.news_group_id = t.news_group_id AND tpn.send_date <= t.send_date )
注：时间相同可能会有误差
原文：http://www.cnblogs.com/zhoucx66/p/5674234.html