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

This forum is recommended for questions regarding administration of PostgreSQL, which your questions are not related
to.pgsql-gernal or pgsql-sql would be better fits. 

But I'd strongly suggest that you pause and learn SQL basics in a structured way. You've said several times that you've
beenwriting queries for a long time, but your questions here demontrate a serious lack of understanding of the basics.
Mypersonal recommendation: 

https://www.postgresql.org/docs/11/tutorial-sql.html

Or, for a quicker start:

https://www.postgresql.org/docs/11/tutorial-sql.html
https://www.postgresql.org/docs/11/sql.html

--
Scott Ribe
scott_ribe@elevated-dev.com
https://www.linkedin.com/in/scottribe/