Use `FOREIGN KEY` in the `CREATE TABLE` statement
Reference the parent table column with `REFERENCES`
Ensure the referenced column is a `PRIMARY KEY` or `UNIQUE` key
Match the data type of the foreign key column with the referenced column
Example:
`CREATE TABLE orders (`
`order_id INT PRIMARY KEY,`
`customer_id INT,`
`FOREIGN KEY (customer_id) REFERENCES customers(customer_id)`
`);`
Add a foreign key to an existing table with `ALTER TABLE`
Example:
`ALTER TABLE orders`
`ADD CONSTRAINT fk_customer`
`FOREIGN KEY (customer_id) REFERENCES customers(customer_id);`
Use `ON DELETE` and `ON UPDATE` actions if needed
Example:
`FOREIGN KEY (customer_id) REFERENCES customers(customer_id) ON DELETE CASCADE ON UPDATE CASCADE;`