What is referential integrity?

Referential Integrity is a property of data in a relational database that guarantees that links between tables remain valid. It prevents the following issues:

Deletion of a row from the "parent" table that is referenced by rows in the "child" table.
Changing the primary key in the "parent" table that is referenced by rows in the "child" table, without updating those references accordingly.
Inserting a row into the "child" table with a reference to a non-existent primary key in the "parent" table.

It is implemented using foreign keys (FOREIGN KEY).

Examples of constraints for foreign keys:

ON DELETE CASCADE: When a row in the parent table is deleted, the corresponding rows in the child table are also deleted.
ON DELETE SET NULL: When a row in the parent table is deleted, the corresponding foreign key in the child table is set to NULL.
ON DELETE RESTRICT: Deletion of a row in the parent table is prohibited if there are references to it in the child table.
ON UPDATE CASCADE: When the primary key in the parent table is updated, the corresponding foreign keys in the child table are also updated.

-- Example of a "users" table (parent)
CREATE TABLE users (
    user_id INT PRIMARY KEY,
    username VARCHAR(255) NOT NULL
);

-- Example of an "orders" table (child) with a foreign key
CREATE TABLE orders (
    order_id INT PRIMARY KEY,
    user_id INT,
    order_date DATE,
    FOREIGN KEY (user_id) REFERENCES users(user_id)
        ON DELETE CASCADE /* Example of constraint usage */
        ON UPDATE CASCADE /* Example of constraint usage */
);