Question

我想使用外键来保持完整性并避免孤儿（我已经使用了innoDB）。

如何制作DELETE ON CASCADE的SQL语句？

如果我删除某个类别，那么如何确保它不会删除与其他类别相关的产品。

数据透视表“categories_products”在其他两个表之间创建了多对多关系。

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

products
- id
- name
- price

categories_products
- categories_id
- products_id

Answer 1

如果您的级联删除了产品，因为它是被杀死的类别的成员，那么您已经不正确地设置了外键。根据您的示例表，您应该设置以下表格：

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，您可以删除产品或类别，只有categories_products中的关联记录会同时消失。级联不会在树上向上移动并删除父产品/类别表。

e.g。

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

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

如果你删除'red'类别，那么只有类别表中的'red'条目会死掉，以及两个条目prod / cats：'red boots'和'red coat'。

删除不会进一步级联，也不会删除“靴子”和“大衣”类别。

评论后续：

你仍然误解了级联删除的工作方式。它们仅影响定义“on delete cascade”的表。在这种情况下，级联在“categories_products”表中设置。如果删除“红色”类别，则category_products中将级联删除的唯一记录是category_id = red。它不会触及'category_id = blue'的任何记录，也不会向前传到“products”表，因为该表中没有定义外键。

这是一个更具体的例子：

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）：

DELETE FROM categories WHERE (id = 2);

DBMS将查看所有具有指向'categories'表的外键的表，并删除匹配id为2的记录。因为我们只在products_categories中定义了外键关系，删除完成后，您最终得到此表：

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

products表中没有定义外键，因此级联在那里不起作用，所以你仍然会列出靴子和连指手套。没有'蓝色靴子'，也没有'蓝色手套'了。

Answer 2

我对这个问题的答案感到困惑，所以我在MySQL中创建了一个测试用例，希望这有帮助

-- Schema
CREATE TABLE T1 (
    `ID` int not null auto_increment,
    `Label` varchar(50),
    primary key (`ID`)
);

CREATE TABLE T2 (
    `ID` int not null auto_increment,
    `Label` varchar(50),
    primary key (`ID`)
);

CREATE TABLE TT (
    `IDT1` int not null,
    `IDT2` int not null,
    primary key (`IDT1`,`IDT2`)
);

ALTER TABLE `TT`
    ADD CONSTRAINT `fk_tt_t1` FOREIGN KEY (`IDT1`) REFERENCES `T1`(`ID`) ON DELETE CASCADE,
    ADD CONSTRAINT `fk_tt_t2` FOREIGN KEY (`IDT2`) REFERENCES `T2`(`ID`) ON DELETE CASCADE;

-- Data
INSERT INTO `T1` (`Label`) VALUES ('T1V1'),('T1V2'),('T1V3'),('T1V4');
INSERT INTO `T2` (`Label`) VALUES ('T2V1'),('T2V2'),('T2V3'),('T2V4');
INSERT INTO `TT` (`IDT1`,`IDT2`) VALUES
(1,1),(1,2),(1,3),(1,4),
(2,1),(2,2),(2,3),(2,4),
(3,1),(3,2),(3,3),(3,4),
(4,1),(4,2),(4,3),(4,4);

-- Delete
DELETE FROM `T2` WHERE `ID`=4; -- Delete one field, all the associated fields on tt, will be deleted, no change in T1
TRUNCATE `T2`; -- Can't truncate a table with a referenced field
DELETE FROM `T2`; -- This will do the job, delete all fields from T2, and all associations from TT, no change in T1

Answer 3

我认为（我不确定）外键约束不会完全符合您的表设计。也许最好的办法是定义一个存储过程，按照你想要的方式删除一个类别，然后在你想删除一个类别时调用该过程。

CREATE PROCEDURE `DeleteCategory` (IN category_ID INT)
LANGUAGE SQL
NOT DETERMINISTIC
MODIFIES SQL DATA
SQL SECURITY DEFINER
BEGIN

DELETE FROM
    `products`
WHERE
    `id` IN (
        SELECT `products_id`
        FROM `categories_products`
        WHERE `categories_id` = category_ID
    )
;

DELETE FROM `categories`
WHERE `id` = category_ID;

END

您还需要将以下外键约束添加到链接表中：

ALTER TABLE `categories_products` ADD
    CONSTRAINT `Constr_categoriesproducts_categories_fk`
    FOREIGN KEY `categories_fk` (`categories_id`) REFERENCES `categories` (`id`)
    ON DELETE CASCADE ON UPDATE CASCADE,
    CONSTRAINT `Constr_categoriesproducts_products_fk`
    FOREIGN KEY `products_fk` (`products_id`) REFERENCES `products` (`id`)
    ON DELETE CASCADE ON UPDATE CASCADE

当然，CONSTRAINT子句也可以出现在CREATE TABLE语句中。

创建这些模式对象后，您可以通过发布CALL DeleteCategory(category_ID)（其中category_ID是要删除的类别）来删除类别并获得所需的行为，并且它将按您的需要运行。但是不要发出正常的DELETE FROM查询，除非您想要更多标准行为（即仅从链接表中删除，并单独留下products表）。

MySQL外键约束，级联删除

3 个答案: