MySQL：对添加的JOIN表使用LIMIT

Question

查看

应用程序的视图包含一个page，其中显示了一堆轮播（每个轮播= 一个标签），每个轮播应包含5个products：

+-------------------------------------+
|Page                                 |
|+-----------------------------------+|
||Tag1                               ||
||+--------+ +--------+    +--------+||
|||Product1| |Product2| .. |Product5|||
||+--------+ +--------+    +--------+||
|+-----------------------------------+|
|...                                  |
|+-----------------------------------+|
||TagN                               ||
||...                                ||
+-------------------------------------+

数据库

页面

+-------+-------------+------+-----+---------+----------------+
| Field | Type        | Null | Key | Default | Extra          |
+-------+-------------+------+-----+---------+----------------+
| id    | int(11)     | NO   | PRI | NULL    | auto_increment |
| name  | varchar(16) | NO   | UNI | NULL    |                |
+-------+-------------+------+-----+---------+----------------+

CREATE TABLE `pages` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(16) COLLATE utf8mb4_unicode_ci NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=2 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

pages_tags

+--------------+------------+------+-----+---------+----------------+
| Field        | Type       | Null | Key | Default | Extra          |
+--------------+------------+------+-----+---------+----------------+
| id           | int(11)    | NO   | PRI | NULL    | auto_increment |
| page_id      | int(11)    | NO   | MUL | NULL    |                |
| tag_id       | int(11)    | NO   | MUL | NULL    |                |
| position     | tinyint(4) | NO   |     | NULL    |                |
+--------------+------------+------+-----+---------+----------------+

CREATE TABLE `pages_tags` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `page_id` int(11) NOT NULL,
  `tag_id` int(11) NOT NULL,
  `position` tinyint(4) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `page_id` (`page_id`),
  KEY `tag_id` (`tag_id`),
  CONSTRAINT `pages_tags_ibfk_1` FOREIGN KEY (`page_id`) REFERENCES `pages` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `pages_tags_ibfk_2` FOREIGN KEY (`tag_id`) REFERENCES `tags` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=663 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

标签

+---------+--------------+------+-----+---------+----------------+
| Field   | Type         | Null | Key | Default | Extra          |
+---------+--------------+------+-----+---------+----------------+
| id      | int(11)      | NO   | PRI | NULL    | auto_increment |
| name    | varchar(128) | NO   |     | NULL    |                |
+---------+--------------+------+-----+---------+----------------+

CREATE TABLE `tags` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(128) COLLATE utf8mb4_unicode_ci NOT NULL,
  PRIMARY KEY (`id`),
) ENGINE=InnoDB AUTO_INCREMENT=149 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

products_tags

+------------+---------+------+-----+---------+----------------+
| Field      | Type    | Null | Key | Default | Extra          |
+------------+---------+------+-----+---------+----------------+
| id         | int(11) | NO   | PRI | NULL    | auto_increment |
| product_id | int(11) | NO   | MUL | NULL    |                |
| tag_id     | int(11) | NO   | MUL | NULL    |                |
+------------+---------+------+-----+---------+----------------+

CREATE TABLE `products_tags` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `product_id` int(11) NOT NULL,
  `tag_id` int(11) NOT NULL,
  PRIMARY KEY (`id`),
  KEY `tag_id` (`tag_id`),
  KEY `bouquet_id` (`product_id`),
  CONSTRAINT `products_tags_ibfk_1` FOREIGN KEY (`product_id`) REFERENCES `products` (`id`) ON DELETE CASCADE ON UPDATE CASCADE,
  CONSTRAINT `products_tags_ibfk_2` FOREIGN KEY (`tag_id`) REFERENCES `tags` (`id`) ON DELETE CASCADE ON UPDATE CASCADE
) ENGINE=InnoDB AUTO_INCREMENT=5698 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

产品

+-------------+--------------+------+-----+---------+----------------+
| Field       | Type         | Null | Key | Default | Extra          |
+-------------+--------------+------+-----+---------+----------------+
| id          | int(11)      | NO   | PRI | NULL    | auto_increment |
| name        | varchar(128) | NO   | UNI | NULL    |                |
+-------------+--------------+------+-----+---------+----------------+

CREATE TABLE `products` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(128) COLLATE utf8mb4_unicode_ci NOT NULL,
  PRIMARY KEY (`id`),
  UNIQUE KEY `name` (`name`)
) ENGINE=InnoDB AUTO_INCREMENT=335 DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci

目标

目标是获得一个输出，其中将包含页面上的所有标签，每个标签最多包含5个产品。

查询

这是我卡住之前的查询：

SELECT t.name, p.name 
FROM pages a 
LEFT JOIN pages_tags a_t ON a_t.page_id = a.id 
LEFT JOIN tags t ON t.id = a_t.tag_id 
LEFT JOIN products p ON p.id = (
  SELECT p_t.product_id FROM products_tags p_t WHERE p_t.tag_id = t.id LIMIT 1
);`

问题

1. 无法使用limit 5：

错误1242（21000）：子查询返回多于1行

2. 无法使用in + limit：

错误1235（42000）：此版本的MySQL尚不支持'LIMIT＆IN / ALL / ANY / SOME子查询'

需要帮助。

---

我想退还什么（示例）

• 页面
  • tag1
    • 产品1
    • product2
    • product3
    • product4
    • product5
  • tag2
    • product6
    • product7
    • product8
    • product9
    • product10
  • ...
  • tagN
    • productK
    • productL（但始终为最大值5）

Answer 1

您也许可以使用标量子查询来做您想做的事情：

p.id <= ANY (SELECT p_t.product_id
             FROM products_tags p_t
             WHERE p_t.tag_id = t.id 
             ORDER BY p_t.product_id
             LIMIT 1 OFFSET 4
            )

ANY用于处理少于5个标签的情况。

注意：这假设您要按顺序订购产品ID。