List of clients who haven't made an order

CREATE OR REPLACE FUNCTION get_clients_without_orders()
RETURNS TABLE (
    client_id INT,
    client_name TEXT,
    email TEXT
)
LANGUAGE plpgsql
AS $$
BEGIN
    RETURN QUERY
    SELECT
        c.client_id,
        c.name AS client_name,
        c.email
    FROM client c
    LEFT JOIN makes_order mo
        ON c.client_id = mo.client_id
    WHERE mo.order_num IS NULL
    ORDER BY c.client_id;
END;
$$;

Relational Algebra

• C(client_id, name, first_name, last_name, email, password, delivery_address)
• MO(client_id, order_num)

JOIN clients with their orders:

• J1 ← C ⟕C.client_id = MO.client_id MO

SELECT only clients who aren't connected to any orders:

• F1 ← σorder_num IS NULL(J1)

Sort by client ID:

• R_final ← τclient_id ASC(F1)