Why Does ALTER IGNORE TABLE Still Result in Integrity Constraint Violations in MySQL?

MySQL ALTER IGNORE TABLE Causes Integrity Constraint Violation

Modifying a MySQL table using ALTER IGNORE TABLE to remove duplicate values while adding a unique key should prevent duplicate key errors. However, users have encountered the error "Integrity constraint violation" despite using the IGNORE flag.

Explanation:

The MySQL documentation states that IGNORE instructs the database to skip duplicate rows on unique keys during the ALTER TABLE operation. But in some cases, especially with InnoDB tables, there seems to be a bug in the IGNORE extension that leads to integrity constraint violations.

Solution:

To resolve the issue, consider the following workaround:

1. Switch to MyISAM Engine:

   • Execute the query ALTER TABLE table ENGINE MyISAM; to convert the table to the MyISAM engine.

2. Add Unique Index with IGNORE:

   • Execute the query ALTER IGNORE TABLE table ADD UNIQUE INDEX dupidx (field); to add the unique index while ignoring duplicates.

3. Convert Back to InnoDB:

   • Once the index is added, execute the query ALTER TABLE table ENGINE InnoDB; to revert the table to the InnoDB engine.

Note:

• This solution will work if there are no foreign key constraints on the table. If there are constraints, they must be temporarily removed and re-added after the above steps.
• The workaround may need to be employed for each unique index added to the table that triggers the integrity constraint violation.

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