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