Date: Sun, 21 Jul 2019 14:24:35 +0000 (UTC)
From: Karen Goh <karenworld@yahoo.com>
To: pgsql-admin@lists.postgresql.org, Ron <ronljohnsonjr@gmail.com>
Message-ID: <1745586001.2633224.1563719075867@mail.yahoo.com>
In-Reply-To: <923fb3b1-90e9-e0b5-bc5b-d896128c3f28@gmail.com>
References: <820008379.2579287.1563670685484@mail.yahoo.com>
 <5CE41E80-274A-44DA-9553-E2DCA260F751@jakobs.com>
 <385857992.2643116.1563717238725@mail.yahoo.com>
 <923fb3b1-90e9-e0b5-bc5b-d896128c3f28@gmail.com>
Subject: Re: How do I alter an existing column and add a foreign key which
 is a Primary key to a table?
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On Sunday, July 21, 2019, 10:12:33 PM GMT+8, Ron <ronljohnsonjr@gmail.com> wrote:


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';

Hi Ron,

You can't Select s_tutor.zip_code from subject_tutor as zip_code belongs to s_tutor table.

Hence, I wrote my last message about my confusion.


-- 
Angular momentum makes the world go 'round.


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<div>                <br><br><br><br><br>On Sunday, July 21, 2019, 10:12:33=
 PM GMT+8, Ron &lt;ronljohnsonjr@gmail.com&gt; wrote:<br><br><br>On 7/21/19=
 8:53 AM, Karen Goh wrote:<br>&gt;<br>&gt; On Sunday, July 21, 2019, 3:28:1=
0 PM GMT+8, Holger Jakobs<br>&gt; &lt;holger@jakobs.com&gt; wrote:<br>&gt;<=
br>&gt; Am 21. Juli 2019 02:58:05 MESZ schrieb Karen Goh &lt;karenworld@yah=
oo.com&gt;:<br>&gt;<br>[snip]<br><br>&gt;<br>&gt; This looks like a 1:1 rel=
ationship which is to be avoided.<br>&gt;<br>&gt; One ALTER TABLE command e=
ither adds a primary or a foreign key. There is<br>&gt; no FK without the k=
eyword REFERENCES.<br>&gt;<br>&gt; FK relationships help in keeping the dat=
a consistent, but they are totally<br>&gt; unrelated to/useless for SELECT =
statements and their JOIN operations. This<br>&gt; is a common misconceptio=
n.<br>&gt;<br>&gt; Hi Holger,<br>&gt;<br>&gt; After reading your reply, I a=
m confused now.<br>&gt;<br>&gt; Cos without creating foreign key in my tuto=
r_subject how am I going to<br>&gt; retrieve the zipcode at s_tutor table t=
hat meet the list of subject_name<br>&gt; and tutor_id in tutor_subject ? T=
here should have some reference right ?<br>&gt; If not, how does the databa=
se tell this zipcode belong to which tutor_id ?<br><br><br>The reference is=
 in the SELECT statement, not in the FK definition.<br><br>SELECT s_tutor.z=
ip_code<br>from subject_tutor, s_tutor<br>where s_tutor.tutor_id =3D subjec=
t_tutor.tutor_id<br>and subject_tutor.subject_name =3D &#39;Angela Merkel&#=
39;;<br><br>Hi Ron,<br><br>You can&#39;t Select s_tutor.zip_code from subje=
ct_tutor as zip_code belongs to s_tutor table.<br><br>Hence, I wrote my las=
t message about my confusion.<br><br><br><br>-- <br>Angular momentum makes =
the world go &#39;round.<br><br><br><br>            </div>           =20
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