ALTER TABLE时，SQL键列不存在

Question

ALTER TABLE book
ADD FOREIGN KEY (category_id)
REFERENCES category (category_id)
ON DELETE CASCADE
ON UPDATE CASCADE;

每当我运行此ALTER TABLE语句时，它总是返回“错误代码：1072。键列'category_id'不存在”。

我敢肯定这是一个简单的解决方法，但是我在向一个表中添加外键时遇到了问题。 我上周刚开始学习SQL，因此任何帮助将不胜感激！

我的CREATE TABLE语句如下：

CREATE TABLE IF NOT EXISTS book
(book_id INT not null PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(100) not null,
description TEXT not null,
price DEC(18,2) not null
) ENGINE=InnoDB;

CREATE TABLE IF NOT EXISTS category
(category_id INT not null PRIMARY KEY AUTO_INCREMENT,
code1 VARCHAR(20) not null,
name1 VARCHAR(100) not null
) ENGINE=InnoDB;

Answer 1

CREATE TABLE IF NOT EXISTS category
(category_id INT not null PRIMARY KEY AUTO_INCREMENT,
code1 VARCHAR(20) not null,
name1 VARCHAR(100) not null
) ENGINE=InnoDB;

该表具有Primay键category_id，但下表没有在Alter命令中引用的列为“ category_id”。在下表中添加列“ category_id”，然后执行Alter命令。

CREATE TABLE IF NOT EXISTS book
(book_id INT not null PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(100) not null,
description TEXT not null,
price DEC(18,2) not null
) ENGINE=InnoDB;

Answer 2

这是完整的脚本，然后

CREATE TABLE IF NOT EXISTS book
(book_id INT not null PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(100) not null,
description TEXT not null,
price DEC(18,2) not null,
category_id INT not null 
) ENGINE=InnoDB;

CREATE TABLE IF NOT EXISTS category
(category_id INT not null PRIMARY KEY AUTO_INCREMENT,
code1 VARCHAR(20) not null,
name1 VARCHAR(100) not null
) ENGINE=InnoDB;

ALTER TABLE book
ADD FOREIGN KEY (category_id)
REFERENCES category (category_id)
ON DELETE CASCADE
ON UPDATE CASCADE;