Обсуждение: 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

On Fri, May 6, 2011 at 9:50 PM, Grzegorz Szpetkowski
<gszpetkowski@gmail.com> wrote:
> 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
[...]
>
> 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
> );

I don't think we should change the example, but we could probably
clarify the wording.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Robert Haas wrote:
> On Fri, May 6, 2011 at 9:50 PM, Grzegorz Szpetkowski
> <gszpetkowski@gmail.com> wrote:
> > 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
> [...]
> >
> > 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
> > );
>
> I don't think we should change the example, but we could probably
> clarify the wording.

Any ideas on how to clarify the wording?

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +

On Sun, Sep 11, 2011 at 9:09 PM, Bruce Momjian <bruce@momjian.us> wrote:
> Robert Haas wrote:
>> On Fri, May 6, 2011 at 9:50 PM, Grzegorz Szpetkowski
>> <gszpetkowski@gmail.com> wrote:
>> > 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
>> [...]
>> >
>> > 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
>> > );
>>
>> I don't think we should change the example, but we could probably
>> clarify the wording.
>
> Any ideas on how to clarify the wording?

Maybe something like this:

Now every product_no that appears in the orders table must also appear
in the products table.  Foreign key constraints are not checked for
NULL values, so product_no may also be NULL; we could prohibit this by
declaring the column NOT NULL.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

Robert Haas wrote:
> On Fri, May 6, 2011 at 9:50 PM, Grzegorz Szpetkowski
> <gszpetkowski@gmail.com> wrote:
> > 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
> [...]
> >
> > 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
> > );
>
> I don't think we should change the example, but we could probably
> clarify the wording.

Done with the attached patch.

--
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +
diff --git a/doc/src/sgml/ddl.sgml b/doc/src/sgml/ddl.sgml
new file mode 100644
index c624fc2..ea840fb
*** a/doc/src/sgml/ddl.sgml
--- b/doc/src/sgml/ddl.sgml
*************** CREATE TABLE orders (
*** 688,694 ****
      quantity integer
  );
  </programlisting>
!     Now it is impossible to create orders with
      <structfield>product_no</structfield> entries that do not appear in the
      products table.
     </para>
--- 688,694 ----
      quantity integer
  );
  </programlisting>
!     Now it is impossible to create orders with non-NULL
      <structfield>product_no</structfield> entries that do not appear in the
      products table.
     </para>