Топ-100
 

FOREIGN KEY

База данных: MySQL
Текст

MySQL supports foreign keys, which permit cross-referencing related
data across tables, and foreign key constraints, which help keep the
related data consistent.

A foreign key relationship involves a parent table that holds the
initial column values, and a child table with column values that
reference the parent column values. A foreign key constraint is defined
on the child table.

The essential syntax for a defining a foreign key constraint in a
CREATE TABLE or ALTER TABLE statement includes the following:

[CONSTRAINT [symbol]] FOREIGN KEY
[index_name] (col_name, ...)
REFERENCES tbl_name (col_name,...)
[ON DELETE reference_option]
[ON UPDATE reference_option]

reference_option:
RESTRICT | CASCADE | SET NULL | NO ACTION | SET DEFAULT

URL: https://dev.mysql.com/doc/refman/8.0/en/create-table-foreign-keys.html

Пример

CREATE TABLE product (
category INT NOT NULL, id INT NOT NULL,
price DECIMAL,
PRIMARY KEY(category, id)
) ENGINE=INNODB;

CREATE TABLE customer (
id INT NOT NULL,
PRIMARY KEY (id)
) ENGINE=INNODB;

CREATE TABLE product_order (
no INT NOT NULL AUTO_INCREMENT,
product_category INT NOT NULL,
product_id INT NOT NULL,
customer_id INT NOT NULL,

PRIMARY KEY(no),
INDEX (product_category, product_id),
INDEX (customer_id),

FOREIGN KEY (product_category, product_id)
REFERENCES product(category, id)
ON UPDATE CASCADE ON DELETE RESTRICT,

FOREIGN KEY (customer_id)
REFERENCES customer(id)
) ENGINE=INNODB;