CREATE TABLE discount_group (
        customer_group_id INTEGER DEFAULT NULL,
        customer_id INTEGER DEFAULT NULL,
        product_group_id INTEGER DEFAULT NULL,
        product_id INTEGER DEFAULT NULL,
        discount REAL NOT NULL
);

ALTER TABLE discount_group
ADD CONSTRAINT customer_group_discount_group FOREIGN KEY (customer_group_id)
REFERENCES customer_group(id) ON UPDATE CASCADE ON DELETE CASCADE;

ALTER TABLE discount_group
ADD CONSTRAINT customer_discount_group FOREIGN KEY (customer_id)
REFERENCES customer(id) ON UPDATE CASCADE ON DELETE CASCADE;

ALTER TABLE discount_group
ADD CONSTRAINT product_group_discount_group FOREIGN KEY (product_group_id)
REFERENCES product_group(id) ON UPDATE CASCADE ON DELETE CASCADE;

ALTER TABLE discount_group
ADD CONSTRAINT product_discount_group FOREIGN KEY (product_id)
REFERENCES product(id) ON UPDATE CASCADE ON DELETE CASCADE;



CREATE OR REPLACE FUNCTION add_discount(
        customer_group_id INTEGER,
        customer_id INTEGER,
        product_group_id INTEGER,
        product_id INTEGER,
        discount REAL
)
RETURNS INTEGER AS $$
DECLARE points INTEGER DEFAULT 0;
BEGIN
    IF customer_group_id IS NOT NULL OR customer_id IS NOT NULL THEN
        points := points + 1;
    END IF;
    IF product_group_id IS NOT NULL OR product_id IS NOT NULL THEN
        points := points +1;
    END IF;

    IF points < 2 THEN
        RAISE EXCEPTION 'Either no customer_group_id/customer_id
                         or product_group_id/product_id was specified';
    END IF;
    RETURN 0;
END;
$$ LANGUAGE plpgsql;