Hello All,

I have created the following table:
CREATE TABLE `kuku` (
`a` int(11) NOT NULL DEFAULT '0',
`b` varchar(20) DEFAULT NULL,
`c` int(11) DEFAULT NULL,
PRIMARY KEY (`a`),
UNIQUE KEY `aaa` (`c`)
) ENGINE=MyISAM DEFAULT CHARSET=latin1

and tried:
insert into kuku values (1, 'aaaa', 1);
insert into kuku values (2, 'aaaa', 1); - alert with duplicate error message
insert into kuku values (3, 'aaaa', null);
insert into kuku values (4, 'aaaa', null); - no alert was found

database - 0 , NULL, empty string, Default values - MySql Questions - Stack Overflow

NULL isn't a comparable value you have to use the specific verbs to look at nulls in MySQL or for that matter virtually every other database

Because mo two null values are equal.You can refer to MySql select with NULL value - MySQL tutorial | w3resource for learning NULL in detail.