title: Model a Zero or One to Many Relationship
body: <p>How should I model a zero or one to a many relationship in the database?   For example, a user record may or may not have a parent.   So should my user table have a t_user.parent_id or should I have an associative table called t_user_hierarchy with the columns t_user_hierarchy.parent_id and t_user_hierarchy.user_id?</p>