【思路】-分页-双top分页算法的原理

描述：实现分页的一种算法

大致过程：访客访问不同的分页，为这个当前页生成动态的查询SQL，然后送到数据库中执行

输入：总条数，每页多少条，第几页，查询的SQL，排序的字段

注意：传入的排序字段需要构成唯一记录，这就意味着不能单独的使用SortOrder这个单独排序

字段反转：SortOrder asc,PostID desc；反转之后是SortOrder desc,PostID asc；

实现过程

访问第一页

访问最后一页

访问前半数分页

访问后半数分页

100条数据 每页显示4条 按照ID降序 排序

当取第一页时：直接对所有数据降序取出前4条

当取第二页时：

1.先按降序获得前8条数据，然后升序排列着8条数据，

2.这样第二页的数据直接跑到了前面，取出4条，

3.然后在倒叙回去就是第二页的数据

同样的以此往下类推

当后半数分页时：

当取最后一页时：将所有数据升序过来，取出前4条即可

观察生成的过程

访问前半数分页

2016-06-17 08:48:34  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc LIMIT 4

2016-06-17 08:48:43  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 8  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:48:44  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 12  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:48:45  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 16  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:48:45  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 20  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:48:46  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 24  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:48:46  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 28  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:48:47  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 32  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:48:47  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 36  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:48:48  SELECT * FROM  (  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID desc  LIMIT 40  ) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

访问后半数分页

2016-06-17 08:50:51  SELECT * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 4  ) AS T  ORDER BY PostID desc 

2016-06-17 08:51:02  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 5  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:03  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 9  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:03  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 13  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:04  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 17  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:06  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 21  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:07  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 25  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:07  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 29  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:08  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 33  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:09  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 37  ) AS T  ORDER BY PostID desc  LIMIT 4 

2016-06-17 08:51:09  SELECT  * FROM  (  SELECT  *  FROM (SELECT * FROM Post) AS T  ORDER BY PostID ASC  LIMIT 41  ) AS T  ORDER BY PostID desc  LIMIT 4 

来自为知笔记(Wiz)