Subject: Re: How do I alter an existing column and add a foreign key which is
 a Primary key to a table?
To: pgsql-admin@lists.postgresql.org
References: <820008379.2579287.1563670685484@mail.yahoo.com>
 <5CE41E80-274A-44DA-9553-E2DCA260F751@jakobs.com>
 <385857992.2643116.1563717238725@mail.yahoo.com>
From: Ron <ronljohnsonjr@gmail.com>
Message-ID: <923fb3b1-90e9-e0b5-bc5b-d896128c3f28@gmail.com>
Date: Sun, 21 Jul 2019 09:12:12 -0500
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On 7/21/19 8:53 AM, Karen Goh wrote:
>
> On Sunday, July 21, 2019, 3:28:10 PM GMT+8, Holger Jakobs 
> <holger@jakobs.com> wrote:
>
> Am 21. Juli 2019 02:58:05 MESZ schrieb Karen Goh <karenworld@yahoo.com>:
>
[snip]
>
> This looks like a 1:1 relationship which is to be avoided.
>
> One ALTER TABLE command either adds a primary or a foreign key. There is 
> no FK without the keyword REFERENCES.
>
> FK relationships help in keeping the data consistent, but they are totally 
> unrelated to/useless for SELECT statements and their JOIN operations. This 
> is a common misconception.
>
> Hi Holger,
>
> After reading your reply, I am confused now.
>
> Cos without creating foreign key in my tutor_subject how am I going to 
> retrieve the zipcode at s_tutor table that meet the list of subject_name 
> and tutor_id in tutor_subject ? There should have some reference right ? 
> If not, how does the database tell this zipcode belong to which tutor_id ?

The reference is in the SELECT statement, not in the FK definition.

SELECT s_tutor.zip_code
from subject_tutor, s_tutor
where s_tutor.tutor_id = subject_tutor.tutor_id
and subject_tutor.subject_name = 'Angela Merkel';

-- 
Angular momentum makes the world go 'round.