MySQL外键约束，级联删除

问题描述

我想使用外键来保持完整性并避免孤儿(我已经使用了 innoDB).

I want to use foreign keys to keep the integrity and avoid orphans (I already use innoDB).

我如何制作一个 DELETE ON CASCADE 的 SQL 语句?

How do I make a SQL statment that DELETE ON CASCADE?

如果我删除一个类别，那么我如何确保它不会删除与其他类别相关的产品.

If I delete a category then how do I make sure that it would not delete products that also are related to other categories.

数据透视表categories_products"在另外两个表之间创建多对多关系.

The pivot table "categories_products" creates a many-to-many relationship between the two other tables.

categories
- id (INT)
- name (VARCHAR 255)

products
- id
- name
- price

categories_products
- categories_id
- products_id

推荐答案

如果您的级联删除了一个产品，因为它是被杀死的类别的成员，那么您设置的外键不正确.鉴于您的示例表，您应该具有以下表设置:

If your cascading deletes nuke a product because it was a member of a category that was killed, then you've set up your foreign keys improperly. Given your example tables, you should have the following table setup:

CREATE TABLE categories (
    id int unsigned not null primary key,
    name VARCHAR(255) default null
)Engine=InnoDB;

CREATE TABLE products (
    id int unsigned not null primary key,
    name VARCHAR(255) default null
)Engine=InnoDB;

CREATE TABLE categories_products (
    category_id int unsigned not null,
    product_id int unsigned not null,
    PRIMARY KEY (category_id, product_id),
    KEY pkey (product_id),
    FOREIGN KEY (category_id) REFERENCES categories (id)
       ON DELETE CASCADE
       ON UPDATE CASCADE,
    FOREIGN KEY (product_id) REFERENCES products (id)
       ON DELETE CASCADE
       ON UPDATE CASCADE
)Engine=InnoDB;

这样，您可以删除一个产品或一个类别，并且只有categories_products 中的关联记录会一起消失.级联不会在树上进一步移动并删除父产品/类别表.

This way, you can delete a product OR a category, and only the associated records in categories_products will die alongside. The cascade won't travel farther up the tree and delete the parent product/category table.

例如

products: boots, mittens, hats, coats
categories: red, green, blue, white, black

prod/cats: red boots, green mittens, red coats, black hats

如果删除 'red' 类别，则只有类别表中的 'red' 条目以及 prod/cats 的两个条目:'red boots' 和 'redcoats' 消失.

If you delete the 'red' category, then only the 'red' entry in the categories table dies, as well as the two entries prod/cats: 'red boots' and 'red coats'.

删除不会进一步级联，也不会删除靴子"和外套"类别.

The delete will not cascade any farther and will not take out the 'boots' and 'coats' categories.

评论跟进:

您仍然误解了级联删除的工作原理.它们只影响定义了删除级联"的表.在这种情况下，级联设置在categories_products"表中.如果删除红色"类别，则categories_products 中唯一会级联删除的记录是那些category_id = red 的记录.它不会触及任何 'category_id = blue' 的记录，也不会前进到products"表，因为该表中没有定义外键.

you're still misunderstanding how cascaded deletes work. They only affect the tables in which the "on delete cascade" is defined. In this case, the cascade is set in the "categories_products" table. If you delete the 'red' category, the only records that will cascade delete in categories_products are those where category_id = red. It won't touch any records where 'category_id = blue', and it would not travel onwards to the "products" table, because there's no foreign key defined in that table.

这是一个更具体的例子:

Here's a more concrete example:

categories:     products:
+----+------+   +----+---------+
| id | name |   | id | name    |
+----+------+   +----+---------+
| 1  | red  |   | 1  | mittens |
| 2  | blue |   | 2  | boots   |
+---++------+   +----+---------+

products_categories:
+------------+-------------+
| product_id | category_id |
+------------+-------------+
| 1          | 1           | // red mittens
| 1          | 2           | // blue mittens
| 2          | 1           | // red boots
| 2          | 2           | // blue boots
+------------+-------------+

假设您删除了类别 #2(蓝色):

Let's say you delete category #2 (blue):

DELETE FROM categories WHERE (id = 2);

DBMS 会查看所有有外键指向 'categories' 表的表，并删除匹配 id 为 2 的记录.因为我们只在 products_categories，一旦删除完成，你就会得到这个表:

+------------+-------------+
| product_id | category_id |
+------------+-------------+
| 1          | 1           | // red mittens
| 2          | 1           | // red boots
+------------+-------------+