Fw: How do I alter an existing column and add a foreign key which is a Primary key to a table?

Hope that this is the right place to ask about this regarding how Ron suggested to do a Select something when it is not inside a table at all. He said this is something basic. So hope someone could clarify.

Thanks
----- Forwarded Message ----- From: Ron <ronljohnsonjr(at)gmail(dot)com>Cc: "pgsql-admin(at)lists(dot)postgresql(dot)org" <pgsql-admin(at)lists(dot)postgresql(dot)org>Sent: Sunday, July 21, 2019, 11:37:34 PM GMT+8Subject: Re: How do I alter an existing column and add a foreign key which is a Primary key to a table?
On 7/21/19 10:23 AM, Karen Goh wrote:
>
> On Sunday, July 21, 2019, 11:13:58 PM GMT+8, Ron <ronljohnsonjr(at)gmail(dot)com>
> wrote:
>
>
> On 7/21/19 10:10 AM, Karen Goh wrote:
[snip]
> >
> > Here is a compressed format :
> >
> > s_tutor
> > --------
> > zipcode(text)
> > tutor_id (I just checked this is becomes a 'serial id ?' based on the
> > generated sql
> >
> > ALTER TABLE public.s_tutor ALTER COLUMN tutor_id ADD GENERATED BY DEFAULT
> > AS IDENTITY (
> > SEQUENCE NAME public.tutor_strtutorid_seq
> > START WITH 1
> > INCREMENT BY 1
> > NO MINVALUE
> > NO MAXVALUE
> > CACHE 1
> > );
> >
> > tutor_subject
> > ----------------
> > tutor_id
> > subject_Names
>
>
> select s_tutor.zipcode
> from s_tutor, tutor_subject
> where s_tutor.tutor_id = tutor_subject.tutor_id
> and tutor_subject.subject_names ='Angela Merkel'
>
> Hi Ron,
>
> Could you explain why you select s_tutor.zipcode from tutor_subject since
> there is no zipcode inside tutor_subject?
>

I did not do what you think I did.

> Perhaps my reasoning is wrong and I really want to have your expert view.

It might sound harsh, but is not meant to be: this list is for administering
PostgreSQL, not teaching basic SQL.

--
Angular momentum makes the world go 'round.