Mysql-查询层次结构中仅达到第二级的项目答案

【问题标题】：Mysql- Query items that are up to only 2nd level in hierarchyMysql-查询层次结构中仅达到第二级的项目
【发布时间】：2016-11-29 16:32:18
【问题描述】：

我有一个类别表，我将其解析为另一个表 categories_to_categories 用于 many-to-many 关系。现在我只想选择那些有

parent_id =0
他们父母的 parent_id =0

这是我的Fiddle，在结果集中，我有2个问题，

结果重复
定制衬衫类别不应该存在，因为它已经有一个父级并且它的父级没有parent_id =0

DDL：

CREATE TABLE `categories` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `uuid` varchar(25) DEFAULT NULL,
  `name` varchar(255) DEFAULT NULL,
  `description` text,
  `status` enum('ACTIVE','INACTIVE') DEFAULT 'ACTIVE',
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  `banner_path` varchar(255) DEFAULT NULL,
  `franchise_id` int(11) DEFAULT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `categories_uuid_uindex` (`uuid`)
);



INSERT INTO `categories` VALUES (1,'xyz','Printing','test','ACTIVE','2016-11-29 13:54:15','2016-11-29 13:54:18',NULL,3),(2,'abc','Digital','test','ACTIVE','2016-11-29 14:33:48','2016-11-29 14:33:53',NULL,3),(3,'def','Video','test','ACTIVE','2016-11-29 14:34:25','2016-11-29 14:34:29',NULL,3),(4,'s','Merchandise printing','test','ACTIVE','2016-11-29 14:35:02','2016-11-29 14:35:04',NULL,3),(5,'4','D/C','test','ACTIVE','2016-11-29 14:35:24','2016-11-29 14:35:27',NULL,3),(6,'2','Goods','test','ACTIVE','2016-11-29 14:35:49','2016-11-29 14:35:51',NULL,3),(8,'5','B/A Templates','test','ACTIVE','2016-11-29 14:36:26','2016-11-29 14:36:28',NULL,3),(9,'gggg','Customized Shirts','test','ACTIVE','2016-11-29 14:37:00','2016-11-29 14:37:01',NULL,3);



CREATE TABLE `categories_to_categories` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `category_id` int(11) DEFAULT NULL,
  `parent_id` int(11) DEFAULT NULL,
  `created_at` timestamp NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
  `updated_at` timestamp NOT NULL DEFAULT '0000-00-00 00:00:00',
  PRIMARY KEY (`id`)
);



INSERT INTO `categories_to_categories` VALUES (1,1,0,'2016-11-29 13:54:56','2016-11-29 13:54:59'),(2,2,0,'2016-11-29 14:38:10','2016-11-29 14:38:17'),(3,3,0,'2016-11-29 14:38:28','2016-11-29 14:38:29'),(4,4,1,'2016-11-29 14:38:48','2016-11-29 14:38:51'),(5,5,2,'2016-11-29 14:39:28','2016-11-29 14:39:30'),(6,6,0,'2016-11-29 14:39:41','2016-11-29 14:39:43'),(7,4,6,'2016-11-29 14:39:52','2016-11-29 14:39:55'),(8,7,1,'2016-11-29 14:40:11','2016-11-29 14:40:17'),(9,8,4,'2016-11-29 14:40:29','2016-11-29 14:40:32'),(10,9,2,'2016-11-29 14:40:40','2016-11-29 14:40:42');

不工作的查询：

SELECT * 
  FROM CATEGORIES                     A
 INNER JOIN CATEGORIES_TO_CATEGORIES  B
  ON A.ID = B.ID
 AND B.PARENT_ID = 0
  OR (SELECT PARENT_ID
        FROM CATEGORIES_TO_CATEGORIES
       WHERE ID = B.PARENT_ID
     ) = 0 ;

【问题讨论】：

请将所有必要信息直接添加到问题中。
实际上，'Customized Shirts' 应该在结果集中——它只有一个父节点'Digital'，它是根类别（parent_id = 0）
从 sql fiddle 添加代码

标签： php mysql join relational-database

【解决方案1】：

一种可能的解决方案是使用根类别及其子类别的联合：

SELECT c.id, c.name FROM categories c 
WHERE c.id IN (
  SELECT cc.category_id FROM categories_to_categories cc 
  WHERE cc.parent_id = 0 

  UNION

  SELECT pc.category_id FROM categories_to_categories cc 
  LEFT JOIN categories_to_categories pc ON cc.category_id = pc.parent_id 
  WHERE cc.parent_id = 0 
)
ORDER BY c.id;

此查询的结果将是没有重复的根类别/子类别的列表（“商品印刷”是“印刷”和“商品”的子节点，但在结果集中它只出现一次）：

+----+----------------------+
| id | name                 |
+----+----------------------+
|  1 | Printing             |
|  2 | Digital              |
|  3 | Video                |
|  4 | Merchandise printing |
|  5 | D/C                  |
|  6 | Goods                |
|  9 | Customized Shirts    |
+----+----------------------+

【讨论】：