MySQL: ALTER IGNORE TABLE gives "Integrity constraint violation"

The IGNORE keyword extension to MySQL seems to have a bug in the InnoDB version on some version of MySQL.

You could always, convert to MyISAM, IGNORE-ADD the index and then convert back to InnoDB

ALTER TABLE table ENGINE MyISAM;
ALTER IGNORE TABLE table ADD UNIQUE INDEX dupidx (field);
ALTER TABLE table ENGINE InnoDB;

Note, if you have Foreign Key constraints this will not work, you will have to remove those first, and add them back later.

Or try set session old_alter_table=1 (Don't forget to set it back!)

See: http://mysqlolyk.wordpress.com/2012/02/18/alter-ignore-table-add-index-always-give-errors/

The problem is that you have duplicate data in the field you're trying to index. You'll need to remove the offending duplicates before you can add a unique index.

One way is to do the following:

   CREATE TABLE tmp_table LIKE table;
   ALTER IGNORE TABLE tmp_table ADD UNIQUE INDEX dupidx (field);
   INSERT IGNORE INTO tmp_table SELECT * FROM table;
   DROP TABLE table;
   RENAME TABLE tmp_table TO table;

this allows you to insert only the unique data into the table