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
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
