FOREIGN KEY

The FOREIGN KEY constraint establishes a relationship between two tables. It ensures that the values in a column (or a group of columns) in one table match the values in a column (or a group of columns) in another table. This constraint is used to enforce referential integrity between tables. For example, consider two tables orders and customers, where the orders table has a column customer_id that references the customer_id column in the customers table. The FOREIGN KEY constraint can be applied as follows:

CREATE TABLE customers (
    customer_id INT PRIMARY KEY,
    first_name VARCHAR(50),
    last_name VARCHAR(50)
);

CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    customer_id INT,
    order_date DATE,
    FOREIGN KEY (customer_id) REFERENCES customers(customer_id)
);

In this case, the customer_id column in the orders table must match a customer_id value in the customers table, ensuring that orders are associated with valid customers.

name	type
customer_id	INT
first_name	VARCHAR(50)
last_name	VARCHAR(50)

name	type
order_id	INT
customer_id	INT
order_date	DATE

In the table above, the customer_id column in the orders table references the customer_id column in the customers table, as required by the FOREIGN KEY constraint.