How to set a NOT NULL column into NULL in PostgreSQL? I'm new to PostgreSQL and that is why I'm asking this question. I have a table app_client_users and a column named client_id. Initially I set it as NOT NULL and now I would like to change it to allow NULL. I've tried 

alter table app_client_users alter column client_id int  NULL ;


and

alter table app_client_users alter column client_id   NULL ;


which is not working. How can I fix it?
You can simply drop NOT NULL

ALTER TABLE app_client_users ALTER COLUMN client_id DROP NOT NULL;