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We have a MySQL database with duplicate primary key index entries as a result of a DoS attack. The duplicates cannot be found by selects on the ID (it will only show one row, not two), but if you select by a username or password, it will show the two rows. Is it possible to drop the primary key, remove the duplicate rows and then re-index the primary keys?

We found the duplicates existed after trying to repair the table.

  • I assume this is MyISAM key corruption? Please consider switching to InnoDB. – Rob Olmos Oct 30 '10 at 20:49
  • i think you're right - last night the web hosting company worked some magic on the db and the data was successfully repaired/duplicates removed. (i believe using cPanel) the table and db are indeed using MyISAM. i'll do some research on the benefits of InnoDB, thanks. – taber Oct 31 '10 at 4:11
  • Consider switching to innodb, in careful consulation with the application developers, as part of a properly managed change which is fully tested. Be careful. It is a good idea though, in the broader sense. – MarkR Nov 29 '10 at 22:17
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Some options off the top of my head:

  • Create a new column with a unique constraint, drop the original column, and make the new one your PK.

  • Alternatively, create a new table with a new PK, copy across everything except the duplicate column, drop the original table and rename the new.

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Why don't you make a backup of the table and try it out?

  • the backups won't import due to the duplicate keys – taber Oct 31 '10 at 4:08
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    If you make a mysqldump, you can edit the resulting SQL file. – Peter Stuifzand Nov 1 '10 at 14:34
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If you can delete records from table, the sql command below deletes the youngest duplicate record.

delete from table1 where ID in (select max(ID), username from table1 group by username)

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