To join a table to itself you need to alias the name of the table so the sql parser knows which "instance" of the table you're referring to.
select t1.user, t1.data, t2.user, t2.data from table t1, table t2 where t1.somefield1=t2.somefield2
This is helpful for things like:
create table employees
(name text,
enum text,
title text,
supervisor_enum text);
select
e1.name as emp_name,
e1.enum as emp_enum,
e1.title as emp_title,
e2.name as boss_name,
e2.title as boss_title
from employees e1
left join
employees e2 on
(e1.supervisor_enum=e2.enum);
