ALTER TABLE changes a table to the current
character set. If you get a duplicate-key error during
ALTER TABLE, the cause is either that the
new character sets 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.
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
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
B-. (You may want to move them elsewhere
instead of deleting them.)
ALTER TABLE works in the following way:
Create a new table named
the requested structural changes.
Copy all rows from the original table to
Rename the original table to
your original table name.
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
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 or OS/2,
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.