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

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
theprimary 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)

Please help me on this.

Thanks.