-- 方法1：每组前2条，再截取
SELECT SUBSTRING_INDEX(SUBSTRING_INDEX(t.ids,',',help_topic_id+1),',',-1) AS num
FROM mysql.help_topic,
(SELECT SUBSTRING_INDEX(GROUP_CONCAT(c.id ORDER BY c.date desc, c.title asc), ",", 2) AS ids FROM content c GROUP BY c.type) as t
WHERE help_topic_id < LENGTH(t.ids)-LENGTH(REPLACE(t.ids,',',''))+1;
-- 方法2：每组前2条，再包含
UPDATE content cont,
(SELECT SUBSTRING_INDEX(GROUP_CONCAT(c.id ORDER BY c.date desc, c.title asc), ",", 2) AS ids FROM content c GROUP BY c.type) t
SET cont.state = 0
WHERE find_in_set(cont.id, t.ids);
说明：建表数据就不提供了，就是表content，字段type分组后，每组date倒序、title顺序，取前2条数据，修改state值。
总结：方法1查询较快，但update不好写很慢。方法2查询较慢，但update很快。
另附：函数循环参考
