ALTER TABLE not letting me set NULL or default value?

Question

I am trying to change an existing column in a table I have to allow for null values and then set the default value to null. I tried running the following but it does not seem to be updating the table:

mysql> ALTER TABLE answers_form MODIFY sub_id int unsigned NULL DEFAULT NULL;
Query OK, 0 rows affected (0.00 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> desc answers_form;
+--------------+------------------+------+-----+---------+-------+
| Field        | Type             | Null | Key | Default | Extra |
+--------------+------------------+------+-----+---------+-------+
| answer_id    | int(10) unsigned | NO   | PRI | 0       |       |
| sub_id       | int(10) unsigned | NO   | PRI | 0       |       |
| form_id      | int(10) unsigned | NO   | PRI | NULL    |       |
| value        | varchar(255)     | NO   |     | NULL    |       |
| non_response | bit(1)           | YES  |     | b'0'    |       |
+--------------+------------------+------+-----+---------+-------+
5 rows in set (0.01 sec)

Can anyone see what I am doing wrong here?

Answer 1

its a primary key , mysql doesn't allow any part of the primary key to be null, which does make the fact that it allows a default value of null for the form_id odd, however the docs at

http://dev.mysql.com/doc/refman/5.5/en/optimizing-primary-keys.html

say "Query performance benefits from the NOT NULL optimization, because it cannot include any NULL values".

Just out of curiosity, does it actually allow you to put in null values in the form_id field?