5.3.5. Foreign Keys (The SQL Language) possible enhance

I have some remark about

"Now it is impossible to create orders with product_no entries that do
not appear in the products table."

http://www.postgresql.org/docs/9.0/static/ddl-constraints.html#DDL-CONSTRAINTS-FK

Let' see:

CREATE TABLE products (
product_no integer PRIMARY KEY,
name text,
price numeric
);

CREATE TABLE orders (
order_id integer PRIMARY KEY,
product_no integer REFERENCES products (product_no),
quantity integer
);

INSERT INTO products VALUES (1, 'Bosch vacuum cleaner', 10);
INSERT INTO orders VALUES (1, 1, 1);
INSERT INTO orders VALUES (2, NULL, 5);

There is still possibility to add product_no (exactly NULL) value,
which does not appear (cannot because of primary key nature) in
products table. To get "full solution" you need create orders table as

CREATE TABLE orders (
order_id integer PRIMARY KEY,
product_no integer REFERENCES products (product_no) NOT NULL,
quantity integer
);

Regards,
Grzegorz Szpetkowski