[置顶] MySQL的分层数据管理 无限级分类 设计与优化

     最近做个一基于SQL的无限级分类的目录模块,在网上看到了这个文章,非常不错.

原文是:http://ftp.nchu.edu.tw/MySQL/tech-resources/articles/hierarchical-data.html

http://wenku.baidu.com/view/53c68dd049649b6648d74746.html

     在看下面的无限级分类优化之前,请大家先看看原文先哈!

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1.文章里介绍了常见的基于parent_id的邻接表模型:

 CREATE TABLE category( category_id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(20) NOT NULL, parent INT DEFAULT NULL );

 +-------------+----------------------+--------+ | category_id | name | parent | +-------------+----------------------+--------+ | 1 | ELECTRONICS | NULL | | 2 | TELEVISIONS | 1 | | 3 | TUBE | 2 | | 4 | LCD | 2 | | 5 | PLASMA | 2 | | 6 | PORTABLE ELECTRONICS | 1 | | 7 | MP3 PLAYERS | 6 | | 8 | FLASH | 7 | | 9 | CD PLAYERS | 6 | | 10 | 2 WAY RADIOS | 6 | +-------------+----------------------+--------+

和基于"先序遍历算法"的嵌套集合(Nested Set)模型:  

 CREATE TABLE nested_category ( category_id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(20) NOT NULL, lft INT NOT NULL, rgt INT NOT NULL );

 +-------------+----------------------+-----+-----+ | category_id | name | lft | rgt | +-------------+----------------------+-----+-----+ | 1 | ELECTRONICS | 1 | 20 | | 2 | TELEVISIONS | 2 | 9 | | 3 | TUBE | 3 | 4 | | 4 | LCD | 5 | 6 | | 5 | PLASMA | 7 | 8 | | 6 | PORTABLE ELECTRONICS | 10 | 19 | | 7 | MP3 PLAYERS | 11 | 14 | | 8 | FLASH | 12 | 13 | | 9 | CD PLAYERS | 15 | 16 | | 10 | 2 WAY RADIOS | 17 | 18 | +-------------+----------------------+-----+-----+ 

2.分析与点评 

上述两种算法我个人觉得各和优点,在页面上的类目,在web网站里,最常见的场景是
        1."检索节点的直接子节点"
        2."检索完整的子树"
场景PK:
 1."检索节点的直接子节点"
         就是查找一个目录的直接下级元素,如查询'PORTABLE ELECTRONICS'的直接下级元素:
         对于"基于parent_id的邻接表模型",直接
           "SELECT id,name FROM category WHERE parent_id = 6;"
         查找特定parent_id的所有元素就可以了.
         对于"嵌套集合(Nested Set)模型",按原文的方法可复杂了:

 SELECT node.name, (COUNT(parent.name) - (sub_tree.depth + 1)) AS depth FROM nested_category AS node, nested_category AS parent, nested_category AS sub_parent, ( SELECT node.name, (COUNT(parent.name) - 1) AS depth FROM nested_category AS node, nested_category AS parent WHERE node.lft BETWEEN parent.lft AND parent.rgt AND node.name = 'PORTABLE ELECTRONICS' GROUP BY node.name ORDER BY node.lft )AS sub_tree WHERE node.lft BETWEEN parent.lft AND parent.rgt AND node.lft BETWEEN sub_parent.lft AND sub_parent.rgt AND sub_parent.name = sub_tree.name GROUP BY noe.name HAVING depth <= 1 ORDER BY node.lft;

    这可是最常见的场景,我相信"嵌套集合"这里的性能不会很好,这里"邻接表模型"性能好很多!　

 2."检索完整的子树"
       如查询以"PORTABLE ELECTRONICS"为根的子树
       对于"基于parent_id的邻接表模型",很复杂,涉及到递归操作,用客户端代码会很复杂,用存储过程还是一样递归搜索,性能实在不行. 

       对于"嵌套集合(Nested Set)模型",相当简单:
        "SELECT id,name,parent_id FROM category WHERE lft BETWEEN 10 AND 19 ORDER BY lft" 
   这里"嵌套集合模型"性能好很多!

3.无限级分类优化　 　

　能不能整合"邻接表模型"和"嵌套集合模型"呢？我们试试看

　

    CREATE TABLE category (
      id INT AUTO_INCREMENT PRIMARY KEY,
      name VARCHAR(20) NOT NULL,
      lft INT NOT NULL,
      rgt INT NOT NULL,
      parent_id INT
    );

        


    表面看上去只是简单的数据整合,实际上述两种模式的功能都整合起来了,
    对于1."检索节点的直接子节点"的场景(利用"邻接表模型"的特性): 
        "SELECT id,name FROM category WHERE parent_id = 6;"  
   
    对于2."检索完整的子树"场景(利用"嵌套集合模型"的特性):
　    "SELECT id,name,parent_id FROM category WHERE lft BETWEEN 10 AND 19;"

    这是"邻接表-嵌套集合-混合模型",
    相对于"嵌套集合模型",只是简单地增加了"parent_id"字段,就获得了"邻接表模型"的优点,邻接表与嵌套集合的优点整合,非常不错呢