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

On 7/20/19 8:31 PM, Karen Goh wrote:
>
> On Sunday, July 21, 2019, 9:25:54 AM GMT+8, Ron <ronljohnsonjr@gmail.com> 
> wrote:
>
>
> On 7/20/19 7:58 PM, Karen Goh wrote:
>
> > Hi all,
> >
> > I used to write a script in MYSQL and foreign and primary key will be 
> created.
> >
> > With PG4Admin, I am lost.
> >
> > I realised now that the keys are not created and perhaps that is why the 
> join query is not working out.
> >
> > Please let me know what is the correct way to alter a column in a table 
> to have foreign key to a tutor_id which is also the primary key of that table.
> >
> > So, meaning I need to create a foreign key as well as primary key for 
> tutor_id.
> >
> > So far, this is what I have attempted but it is not working.
> > ALTER TABLE tutor_subject
> > ADD CONSTRAINT tutor_subject_pk
> > PRIMARY KEY (tutor_id)
> > ADD CONSTRAINT tutor_subject_fk
> > FOREIGN KEY (tutor_id)
>
>
> What error message do you get?
>
> Does tutor_id already exist in tutor_subject?
>
> Yes. It is already there but it is the first time I used pgAdmin4 so I 
> just used the add column to put in the infor.
>
> Now, I just tried want to do one thing first which is to alter the 
> tutor_id in tutor_subject to a primary key.
>
> ALTER TABLE tutor_subject
> ADD CONSTRAINT tutor_subject_pk
> PRIMARY KEY (tutor_id)
>
> But, am receiving error messagte :
>
> ERROR: could not create unique index "tutor_subject_pk"
> DETAIL: Key (tutor_id)=(0) is duplicated.
> SQL state: 23505
>
> I noticed several of the rows has 0 at tutor_id. It must have attributed 
> to the table not created properly.
>
> How do I resolve this ? delete those rows?

Naturally.  You can't have a unique index with duplicate keys.


>
> What foreign table are you referencing? (I don't see that referenced in
> your example.)
>
> The foreign table will be s_tutor which has a tutor_id as well.
>
> So, the tutor_id in tutor_subject will be both primary key as well as 
> foreign key.

You can't just say "tutor_id is a foreign key"; you've got to tell it the 
name of the foreign table.

>
> Have you read the documentation?
> https://www.postgresql.org/docs/9.6/sql-altertable.html
> http://www.postgresqltutorial.com/postgresql-primary-key/
> http://www.postgresqltutorial.com/postgresql-foreign-key/
>
>
> -- 
> Angular momentum makes the world go 'round.
>
>
>

-- 
Angular momentum makes the world go 'round.