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