create table nodes (
    node_id integer not null primary key,
    parent  integer references node_id,
    message varchar(1024),
    location varchar(256)
    -- only 256 levels of replies and only 256 replies to a root node
);

-- Let's create a discussion
-- root level
insert into nodes (node_id, message,location) values (1,'Hello','');

-- we count from 'A' to 'Z' in this simplicistic example
insert into nodes (node_id, message,location) values (2,'Re: Hello','A');
insert into nodes (node_id, message,location) values (3,'Re: Hello','B');
insert into nodes (node_id, message,location) values (4,'Re: Re: Hello (1)','AA');
insert into nodes (node_id, message,location) values (5,'Re: Re: Hello (2)','AB');
insert into nodes (node_id, message,location) values (6,'Re: Re: Hello!','BA');