Why Does ALTER IGNORE TABLE Fail with Integrity Constraint Violation Even When Using IGNORE?

MySQL: ALTER IGNORE TABLE Fails with Integrity Constraint Violation

MySQL's ALTER statement allows you to modify table definitions, including adding unique keys. The IGNORE keyword extension enables the removal of duplicate rows during this operation. However, users have reported encountering "Integrity constraint violation" errors even when using IGNORE.

Issue:
When executing an ALTER IGNORE TABLE statement to remove duplicates based on a unique key, an "Integrity constraint violation" error is still encountered, contradicting the documentation's claims that IGNORE prevents this error.

Cause:
The IGNORE keyword extension seems to have a bug in the InnoDB engine on certain versions of MySQL, resulting in the error.

Solution:
To circumvent this issue, you can temporarily convert the table to MyISAM, add the unique index using IGNORE, and then convert it back to InnoDB.

Note: If foreign key constraints are present, you must remove them first and re-add them after the conversion.

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