Bug #70718 Unique index allows duplicates with null values

Description:
Revisit Bug #8173 (http://bugs.mysql.com/bug.php?id=8173).
I am not sure how comments are treated for bugs, which have been closed, so I am opening a new bug report. There were multiple reasons given as to why this is a bug, but lately there were no responses from MySQL developers. Could we have the SQL Standard reviewed again and provide a fix to the problem.

Description of the bug:
unique index allows duplicates if at least one of the columns is null.

How to repeat:
mysql> create table tb
    -> (
    ->  id      int not null auto_increment,
    ->  a       int,
    ->  b       int,
    ->  primary key (id),
    ->  index (a),
    ->  index (b),
    ->  unique index (a,b)
    -> ) type = innodb; /* occurs in myISAM tables as well */
Query OK, 0 rows affected, 1 warning (0.42 sec)

mysql> insert into tb (a,b) values (1,2);
Query OK, 1 row affected (0.00 sec)

mysql> insert into tb (a,b) values (1,2);       /* fails (correct) */
ERROR 1062 (23000): Duplicate entry '1-2' for key 2

mysql> insert into tb (a,b) values (1,null);
Query OK, 1 row affected (0.00 sec)

mysql> insert into tb (a,b) values (1,null);    /* succeeds (incorrect?) */
Query OK, 1 row affected (0.02 sec)

mysql> select version();
+----------------+
| version()      |
+----------------+
| 5.0.0-alpha-nt |
+----------------+
1 row in set (0.00 sec)

Suggested fix:
Return "duplicate key" error.

Hello Dimitriy,

Thank you for the bug report, duplicate of http://bugs.mysql.com/bug.php?id=17825

Thanks,
Umesh