insert null or blank string into a row

From: nawfer <novalidsforspam_at_alt.al>
Date: Sat, 4 Nov 2006 17:59:46 +0100
Message-ID: <ovvuh2ynsvzt$.342sikdvbh6w$.dlg_at_40tude.net>

if the table mysql has 4 field so:

ID      int(11)               NOT NULL auto_increment,
camp2  varchar(10)      NOT NULL default '',
camp3  int(11)             NULL default '',
camp4  varchar(10)      NULL default '',

PRIMARY KEY (ID),
KEY camp3 (camp3)         

why it does not make me to update camp3 and 4 with a value null or '' do not work:
UPDATE $my_var_tabella
SET camp3 = '', camp4 = ''
WHERE ID = $my_var_number

also writing
SET camp3 = NULL, camp4 = NULL not work
where is the error?

instead if I insert some data it works:
SET camp3 = $id_number, camp4 = $name_text

not want '' or NULL, even if with CREATE TABLE I have insert in camp3 and 4 this '' how default Received on Sat Nov 04 2006 - 17:59:46 CET