If you get a duplicate-key error when using ALTER TABLE to change the character set or collation of a character column, the cause is either that the new column collation maps two keys to the same value or that the table is corrupted. In the latter case, you should run REPAIR TABLE on the table.

If ALTER TABLE dies with the following error, the problem may be that MySQL crashed during an earlier ALTER TABLE operation and there is an old table named A-xxx or B-xxx lying around:

Error on rename of './database/name.frm'
to './database/B-xxx.frm' (Errcode: 17)

In this case, go to the MySQL data directory and delete all files that have names starting with A- or B-. (You may want to move them elsewhere instead of deleting them.)

ALTER TABLE works in the following way:

If something goes wrong with the renaming operation, MySQL tries to undo the changes. If something goes seriously wrong (although this shouldn't happen), MySQL may leave the old table as B-xxx. A simple rename of the table files at the system level should get your data back.

If you use ALTER TABLE on a transactional table or if you are using Windows, ALTER TABLE unlocks the table if you had done a LOCK TABLE on it. This is done because InnoDB and these operating systems cannot drop a table that is in use.