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Date: Thu, 15 Sep 2011 06:42:47 -0500
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Subject: Re: 5.3.5. Foreign Keys (The SQL Language) possible enhance
From: Robert Haas <robertmhaas@gmail.com>
To: Bruce Momjian <bruce@momjian.us>
Cc: Grzegorz Szpetkowski <gszpetkowski@gmail.com>, pgsql-docs@postgresql.org
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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