MySQL實現嵌套集合模型

時間  2019-11-06
標籤 mysql 實現 嵌套 集合 模型 欄目 MySQL 简体版
原文   原文鏈接

MySQL實現嵌套集合模型

譯文主要是介紹如何用MySQL來存儲嵌套集合數據。在其中會增長一些本身的理解，也會刪除掉一些自認爲無用的廢話。
這篇文章主要講的是嵌套集合模型，因此鄰接表不是本文的重點，簡單略過就好。node

也許這是原文地址，由於我也不知道這是否是原文。mysql

介紹

什麼是分層數據？
算法

相似於樹形結構，除了根節點和葉子節點外，全部節點都有用一個父節點和多個子節點。sql

那麼，在MySQL中如何處理分層數據呢？函數

原文中介紹了兩種分層結構模型：鄰接表模型和嵌套集合模型。測試

鄰接表模型(The Adjacency List Model)

首先，創建測試表，導入測試數據，網站

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)

在鄰接表中，全部的數據均擁有一個Parent字段，用來存儲它的父節點。當前節點爲根節點的話，它的父節點則爲NULL。
那麼在遍歷的時候，可使用遞歸來實現查詢整棵樹，從根節點開始，不斷尋找子節點（父節點->子節點->父節點->子節點）。3d

檢索分層路徑

通常須要獲取一個分層結構的路徑問題，那麼code

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)

檢索葉子節點

SELECT t1.name FROM
category AS t1 LEFT JOIN category as t2
ON t1.category_id = t2.parent
WHERE t2.category_id IS NULL;

+--------------+
| name         |
+--------------+
| TUBE         |
| LCD          |
| PLASMA       |
| FLASH        |
| CD PLAYERS   |
| 2 WAY RADIOS |
+--------------+

檢索指定路徑

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' AND t4.name = 'FLASH';

+-------------+----------------------+-------------+-------+
| lev1        | lev2                 | lev3        | lev4  |
+-------------+----------------------+-------------+-------+
| ELECTRONICS | PORTABLE ELECTRONICS | MP3 PLAYERS | FLASH |
+-------------+----------------------+-------------+-------+
1 row in set (0.01 sec)

鄰接表的缺點

在檢索路徑的過程當中，除了本層外，每一層都會對應一個LEFT JOIN，那麼若是層數不定怎麼辦？或者層數過多？
在刪除中間層的節點時，須要同時刪除該節點下的全部節點，不然會出現孤立節點。blog

嵌套集合模型Nested Set Model

原文中主要的目的是介紹嵌套集合模型，以下

經過集合的包含關係，嵌套結合模型能夠表示分層結構，每個分層能夠用一個Set來表示（一個圈），父節點所在的圈包含全部子節點所在的圈。

爲了用MySQL來表示集合關係，須要定義連個字段left和right（表示一個集合的範圍）。

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 |
+-------------+----------------------+-----+-----+

因爲left和right是MySQL的保留字，所以，字段名稱用lft和rgt代替。每個集合都是從lft開始到rgt結束，也就是集合的兩個邊界。

在樹中也一樣適用，

當爲樹狀結構編號時，咱們從左到右，一次一層，賦值按照從左到右的順序遍歷其子節點，這種方法稱爲先序遍歷算法。

檢索分層路徑

因爲子節點的lft值總在父節點的lft和rgt值之間，因此能夠經過父節點鏈接到子節點上來檢索整棵樹。

SELECT node.name
FROM nested_category AS node,
        nested_category AS parent
WHERE node.lft BETWEEN parent.lft AND parent.rgt
        AND parent.name = 'ELECTRONICS'
ORDER BY node.lft;

+----------------------+
| name                 |
+----------------------+
| ELECTRONICS          |
| TELEVISIONS          |
| TUBE                 |
| LCD                  |
| PLASMA               |
| PORTABLE ELECTRONICS |
| MP3 PLAYERS          |
| FLASH                |
| CD PLAYERS           |
| 2 WAY RADIOS         |
+----------------------+</pre>

這個方法並不須要考慮層數，並且不須要考慮節點的rgt。

檢索全部葉子節點

因爲每個葉子節點的rgt=lft+1，那麼只須要這一個條件便可。

SELECT name
FROM nested_category
WHERE rgt = lft + 1;

+--------------+
| name         |
+--------------+
| TUBE         |
| LCD          |
| PLASMA       |
| FLASH        |
| CD PLAYERS   |
| 2 WAY RADIOS |
+--------------+

檢索節點路徑

再也不須要多個join鏈接操做。

SELECT parent.name
FROM nested_category AS node,
        nested_category AS parent
WHERE node.lft BETWEEN parent.lft AND parent.rgt
        AND node.name = 'FLASH'
ORDER BY node.lft;

+----------------------+
| name                 |
+----------------------+
| ELECTRONICS          |
| PORTABLE ELECTRONICS |
| MP3 PLAYERS          |
| FLASH                |
+----------------------+

檢索節點深度

經過COUNT和GROUP BY函數來獲取父節點的個數。

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
GROUP BY node.name
ORDER BY node.lft;

+----------------------+-------+
| name                 | depth |
+----------------------+-------+
| ELECTRONICS          |     0 |
| TELEVISIONS          |     1 |
| TUBE                 |     2 |
| LCD                  |     2 |
| PLASMA               |     2 |
| PORTABLE ELECTRONICS |     1 |
| MP3 PLAYERS          |     2 |
| FLASH                |     3 |
| CD PLAYERS           |     2 |
| 2 WAY RADIOS         |     2 |
+----------------------+-------+

甚至能夠獲得分層的縮進結果，

SELECT CONCAT( REPEAT(' ', COUNT(parent.name) - 1), node.name) AS name
FROM nested_category AS node,
        nested_category AS parent
WHERE node.lft BETWEEN parent.lft AND parent.rgt
GROUP BY node.name
ORDER BY node.lft;

+-----------------------+
| name                  |
+-----------------------+
| ELECTRONICS           |
|  TELEVISIONS          |
|   TUBE                |
|   LCD                 |
|   PLASMA              |
|  PORTABLE ELECTRONICS |
|   MP3 PLAYERS         |
|    FLASH              |
|   CD PLAYERS          |
|   2 WAY RADIOS        |
+-----------------------+

檢索子樹的深度

考慮到檢索中須要自鏈接的node或parent，所以須要增長一個額外的鏈接來做爲子查詢來限制子樹。

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 node.name
ORDER BY node.lft;

+----------------------+-------+
| name                 | depth |
+----------------------+-------+
| PORTABLE ELECTRONICS |     0 |
| MP3 PLAYERS          |     1 |
| FLASH                |     2 |
| CD PLAYERS           |     1 |
| 2 WAY RADIOS         |     1 |
+----------------------+-------+

檢索節點的直接子節點

假設一個場景，當用戶點擊網站上電子產品的一個分類時，將呈現該分類下的產品，同時須要列出全部子分類，並非所有分類。
爲了限制顯示分類的層數，須要使用HAVING字句，

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 node.name
HAVING depth &lt;= 1
ORDER BY node.lft;

+----------------------+-------+
| name                 | depth |
+----------------------+-------+
| PORTABLE ELECTRONICS |     0 |
| MP3 PLAYERS          |     1 |
| CD PLAYERS           |     1 |
| 2 WAY RADIOS         |     1 |
+----------------------+-------+

增長新節點

上面已經介紹瞭如何檢索結果，那麼如何才能增長新的節點呢？

若是但願在TELEVISIONS和PROTABLE ELECTRONICS節點之間增長一個新的節點，那麼新節點的lft和rgt的值應該是10和11，那麼全部大於10的節點（新節點右側的節點）的lft和rgt都應該加2，如上圖所示。

LOCK TABLE nested_category WRITE;

SELECT @myRight := rgt FROM nested_category
WHERE name = 'TELEVISIONS';

UPDATE nested_category SET rgt = rgt + 2 WHERE rgt &gt; @myRight;
UPDATE nested_category SET lft = lft + 2 WHERE lft &gt; @myRight;

INSERT INTO nested_category(name, lft, rgt) VALUES('GAME CONSOLES', @myRight + 1, @myRight + 2);

UNLOCK TABLES

若是但願在葉子節點下增長節點，須要修改下查詢語句，

LOCK TABLE nested_category WRITE;

SELECT @myLeft := lft FROM nested_category

WHERE name = '2 WAY RADIOS';

UPDATE nested_category SET rgt = rgt + 2 WHERE rgt &gt; @myLeft;
UPDATE nested_category SET lft = lft + 2 WHERE lft &gt; @myLeft;

INSERT INTO nested_category(name, lft, rgt) VALUES('FRS', @myLeft + 1, @myLeft + 2);

UNLOCK TABLES;```



###刪除節點

刪除葉子節點比較容易，只須要刪除本身，而刪除一箇中間層節點就須要刪除其全部子節點。在這個模型中，全部子節點的節點正好在lft和rgt之間。

LOCK TABLE nested_category WRITE;

SELECT @myLeft := lft, @myRight := rgt, @myWidth := rgt - lft + 1
FROM nested_category
WHERE name = 'GAME CONSOLES';

DELETE FROM nested_category WHERE lft BETWEEN @myLeft AND @myRight;

UPDATE nested_category SET rgt = rgt - @myWidth WHERE rgt > @myRight;
UPDATE nested_category SET lft = lft - @myWidth WHERE lft > @myRight;

UNLOCK TABLES;

在某些狀況下，只須要刪除某個節點，可是並不但願刪除該節點下的子節點數據。
經過把右側全部節點的左右值-2，當前節點的子節點左右值-1

LOCK TABLE nested_category WRITE;

SELECT @myLeft := lft, @myRight := rgt, @myWidth := rgt - lft + 1
FROM nested_category
WHERE name = 'PORTABLE ELECTRONICS';

DELETE FROM nested_category WHERE lft = @myLeft;

UPDATE nested_category SET rgt = rgt - 1, lft = lft - 1 WHERE lft BETWEEN @myLeft AND @myRight;
UPDATE nested_category SET rgt = rgt - 2 WHERE rgt > @myRight;
UPDATE nested_category SET lft = lft - 2 WHERE lft > @myRight;

UNLOCK TABLES;
```

最後的思考

原做者推薦了一本名爲《Joe Celko's Trees and Hierarchies in SQL for Smarties》的書籍，該書的做者是SQL領域的大神Joe Celko（嵌套幾何模型的創造者）。這本書涵蓋了本文中未涉及到的一些高級話題。

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