Dual table default value?
Phil has an interesting test case. But the dual table has one column and one row with the value x. When you select NULL from dual you are selecting a literal from a table. The literal is not actually in the table. That you can select sysdate from dual does not mean 
that sysdate is in the dual table either.


SQL> select sysdate from dual;

SYSDATE
---------------
23-MAY-14