Can’t give PRIMARY KEY property in POSTGRES

Why doesn’t this work exactly?

ALTER TABLE IF EXISTS public.post 
    ALTER COLUMN id TYPE INTEGER PRIMARY KEY
    OWNER to postgres;

The error is

syntax error at or near “PRIMARY”

Answer

The syntax for PostgreSQL is as follows :

Try with this:

ALTER TABLE IF EXISTS public.post 
ADD PRIMARY KEY (id);

Documentation : https://www.postgresqltutorial.com/postgresql-primary-key/