Dual

Returns a constant, and side effects:

1. A NOT IN condition referencing a subquery returning no rows returns all rows:

SELECT 'True' FROM employees
WHERE department_id NOT IN (SELECT 0 FROM DUAL WHERE 1=2);

2. Force sorting in a UNION to get Nth row:

select name_id "id",
	name_last "last",
	rownum "rownum"
  from name
 where name_seriesno = name_now(name_id)
   and (name_last, name_id) in (
       select name_last, name_id
         from name
        where name_serisno = name_now(name_id)
       union
        select 'x', 'y' from dual where 1=2
       )
 group by name_id ,name_last ,rownum
having rownum = 5;

id    last                rownum
----- --------------- ----------
@133  O'Leary                  5