之前在博客里贴过无限分类的介绍，那是官方的例子，但上次在给同事们做介绍的时候，却发现官方的网站已经打不开了。所幸，又找到了一篇，好象这个才是原文？
原文来自：http://www.vbmysql.com/articles/database-design/managing-hierarchical-data-in-mysql
为了这次保证所有的数据都能够记录下来而不是再被茫茫的互联网淹没，所以我做了备份。
mysql.7z
但同样的，我也再次复制一部分的内容下来，好象是说这种无限分类的效率，在被其他项目查询的时候是最高的，但CRUD的时候就有点纠结。
OK，上主菜：
The Nested Set Model
What I would like to focus on in this article is a different approach, commonly referred to as the Nested Set Model. In the Nested Set Model, we can look at our hierarchy in a new way, not as nodes and lines, but as nested containers. Try picturing our electronics categories this way:
Notice how our hierarchy is still maintained, as parent categories envelop their children.We represent this form of hierarchy in a table through the use of left and right values to represent the nesting of our nodes:
CREATE TABLE nested_category ( category_id INT AUTO_INCREMENT PRIMARY KEY, name VARCHAR(20) NOT NULL, lft INT NOT NULL, rgt INT NOT NULL ); INSERT INTO nested_category VALUES(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); SELECT * FROM nested_category ORDER BY category_id; +-------------+----------------------+-----+-----+ | 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 | +-------------+----------------------+-----+-----+
We use lft and rgt because left and right are reserved words in MySQL, see http://dev.mysql.com/doc/mysql/en/reserved-words.html for the full list of reserved words.
So how do we determine left and right values? We start numbering at the leftmost side of the outer node and continue to the right:
This design can be applied to a typical tree as well:
When working with a tree, we work from left to right, one layer at a time, descending to each node’s children before assigning a right-hand number and moving on to the right. This approach is called the modified preorder tree traversal algorithm.
因为我已经在压缩包里做了备份，所以我图片就引用外站的了。
我只贴一种，其余的去看：http://dev.mysql.com/tech-resources/articles/hierarchical-data.html
The Adjacency List Model
Typically the example categories shown above will be stored in a table like the following (I'm including full CREATE and INSERT statements so you can follow along):
CREATE TABLE category(
category_id INT AUTO_INCREMENT PRIMARY KEY,
name VARCHAR(20) NOT NULL,
parent INT DEFAULT NULL);
INSERT INTO category
VALUES(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);
SELECT * FROM category ORDER BY category_id;
+-------------+----------------------+--------+
| 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 |
+-------------+----------------------+--------+
10 rows in set (0.00 sec)
In the adjacency list model, each item in the table contains a pointer to its parent. The topmost element, in this case electronics, has a NULL value for its parent. The adjacency list model has the advantage of being quite simple, it is easy to see that FLASH is a child of mp3 players, which is a child of portable electronics, which is a child of electronics. While the adjacency list model can be dealt with fairly easily in client-side code, working with the model can be more problematic in pure SQL.
Retrieving a Full Tree
The first common task when dealing with hierarchical data is the display of the entire tree, usually with some form of indentation. The most common way of doing this is in pure SQL is through the use of a self-join:
SELECT t1.name AS lev1, t2.name as lev2, t3.name as lev3, t4.name as lev4
FROM category AS t1
LEFT JOIN category AS t2 ON t2.parent = t1.category_id
LEFT JOIN category AS t3 ON t3.parent = t2.category_id
LEFT JOIN category AS t4 ON t4.parent = t3.category_id
WHERE t1.name = 'ELECTRONICS';
+-------------+----------------------+--------------+-------+
| lev1 | lev2 | lev3 | lev4 |
+-------------+----------------------+--------------+-------+
| ELECTRONICS | TELEVISIONS | TUBE | NULL |
| ELECTRONICS | TELEVISIONS | LCD | NULL |
| ELECTRONICS | TELEVISIONS | PLASMA | NULL |
| ELECTRONICS | PORTABLE ELECTRONICS | MP3 PLAYERS | FLASH |
| ELECTRONICS | PORTABLE ELECTRONICS | CD PLAYERS | NULL |
| ELECTRONICS | PORTABLE ELECTRONICS | 2 WAY RADIOS | NULL |
+-------------+----------------------+--------------+-------+
6 rows in set (0.00 sec)
