SQL & PL/SQL

Hi There,
As far as I know, Null is not the same as an empty string; however, when I try this out, I get some unexpected results (well, at least unexpected for my liking):

SQL> CREATE TABLE TS (MID NUMBER,
  2  MDESC VARCHAR2(20) DEFAULT '' NOT NULL);

Table created.

SQL> INSERT INTO TS VALUES(1,'');
INSERT INTO TS VALUES(1,'')
                        *
ERROR at line 1:
ORA-01400: cannot insert NULL into ("TT"."TS"."MDESC")

So, according to the above scenario, I can't insert an empty string!! However, an empty string is a valid string that doesn't have tuples/data!!

How come Oracle translates the null string '' as NULL?


Thanks