A FOREIGN KEY enforces referential integrity by ensuring that a value in one table matches a primary (or unique) key in another table. It prevents βorphanβ records and maintains relationships.
CREATE TABLE orders ( order_id INT AUTO_INCREMENT PRIMARY KEY, customer_id INT, order_date DATE, FOREIGN KEY (customer_id) REFERENCES customers(customer_id) ON DELETE CASCADE ON UPDATE CASCADE );
ON DELETE CASCADE removes orders if the customer is deleted; ON UPDATE CASCADE propagates changes to the key.
ALTER TABLE order_items ADD CONSTRAINT fk_order FOREIGN KEY (order_id) REFERENCES orders(order_id) ON DELETE RESTRICT ON UPDATE CASCADE;
ALTER TABLE order_items DROP FOREIGN KEY fk_order;
fk_table_refTable
).CASCADE
, RESTRICT
, SET NULL
, or NO ACTION
.INNODB
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