MySQL - Can't create table (errno: 150) - Foreign key constraints - mysql

Question

I want to create these 3 tables in my MySQL database:
CREATE TABLE categories (
category_id SMALLINT NOT NULL AUTO_INCREMENT,
category VARCHAR(255) NOT NULL UNIQUE,
description TINYTEXT NOT NULL,
PRIMARY KEY (category_id)
);
CREATE TABLE product (
product_id SMALLINT NOT NULL AUTO_INCREMENT,
product VARCHAR(255) NOT NULL UNIQUE,
description TINYTEXT NOT NULL,
price DECIMAL(6,2) NOT NULL,
PRIMARY KEY (product_id)
);
CREATE TABLE categories_products (
category_id SMALLINT NOT NULL,
product_id SMALLINT NOT NULL,
PRIMARY KEY (category_id, product_id),
FOREIGN KEY (category_id) REFERENCES categories (category_id)
ON DELETE CASCADE ON UPDATE CASCADE,
FOREIGN KEY (product_id) REFERENCES products (product_id)
ON DELETE CASCADE ON UPDATE CASCADE
);
The problem is with the last table. I get an error:
1005 - Can't create table 'test1.categories_products' (errno:
150)
But when i try to create it without the constraints it works.
CREATE TABLE categories_products (
category_id SMALLINT NOT NULL,
product_id SMALLINT NOT NULL,
PRIMARY KEY (category_id, product_id)
);
I need the constraints in case a product or a category is removed. What am i doing wrong?

Answer 1

There seems to be a typo
CREATE TABLE product
(singular)
and then referencing
FOREIGN KEY (product_id) REFERENCES products (product_id)
(plural).

Answer 2

CREATE TABLE categories (
category_id SMALLINT NOT NULL AUTO_INCREMENT,
category VARCHAR(255) NOT NULL UNIQUE,
description TINYTEXT NOT NULL,
PRIMARY KEY (category_id)
);
CREATE TABLE products (
product_id SMALLINT NOT NULL AUTO_INCREMENT,
product VARCHAR(255) NOT NULL UNIQUE,
description TINYTEXT NOT NULL,
price DECIMAL(6,2) NOT NULL,
PRIMARY KEY (product_id)
);
CREATE TABLE categories_products (
category_id SMALLINT NOT NULL,
product_id SMALLINT NOT NULL,
PRIMARY KEY (category_id, product_id),
FOREIGN KEY (category_id) REFERENCES categories (category_id)
ON DELETE CASCADE ON UPDATE CASCADE,
FOREIGN KEY (product_id) REFERENCES products (product_id)
ON DELETE CASCADE ON UPDATE CASCADE
);
You created "product" but referenced "products".
By the way, you should create an ID column and use this one as primary key in "categories_products" and add a second (unique) index on both columns. This is best practice and INNODB does internally create an ID column anyway so...