hi friend,
i do not know why u are getting the error, i did a lot of experiment but not able to get your error. for me all the queries are working.
its very difficult to understand where u have done the mistake. its better u create new table with specific constraints and then insert your data. drop the old table, and then rename the new table to old table name.
i know its not a good idea, but in real time there is not so much of time, and we have to work smarter rather then working hard without any ouput.
sorry for giving a bad suggestion, but sometimes its the reality.
Cheers,
RSY