<p>The error appears when you try to delete rows from a parent table that is referenced by a foreign key in another table without a suitable ON DELETE rule.</p>
<p>MySQL Error 1834: ER_UNUSED5 occurs when you delete rows in a parent table that still have matching rows in a child table. Resolve it by deleting or updating the child rows first, or by adding an ON DELETE CASCADE rule to the foreign key constraint.</p>
Cannot delete rows from table which is parent in a
Error 1834 with the condition name ER_UNUSED5 signals that MySQL blocked a DELETE on a parent table because dependent rows still exist in a child table linked through a foreign key.
The engine enforces referential integrity, so removing a referenced parent row would orphan those child rows. The error first appeared in MySQL 5.7.4.
The most frequent trigger is a foreign key that lacks an ON DELETE CASCADE or SET NULL rule. Without it, MySQL must stop any DELETE that would break the relationship.
Another trigger is application logic that skips deleting child rows before removing the parent record, leaving dangling references.
Option one: delete or update the child rows so no references remain, then retry the parent DELETE.
Option two: alter the foreign key to include ON DELETE CASCADE or SET NULL so MySQL can handle the dependent rows automatically.
An orders table referencing customers often raises this error when code attempts to purge a customer that still owns orders. Deleting those orders first resolves the problem.
Bulk data cleanup scripts frequently fail if they ignore foreign key order; reordering statements or using CASCADE fixes the process.
Design foreign keys with explicit ON DELETE actions that match business rules. Test delete operations in staging to confirm behavior.
Use Galaxy's dependency graph to preview affected tables before executing data-removal queries, reducing surprises in production.
Error 1451 (Cannot delete or update a parent row) is nearly identical but arises in earlier versions; the remedies are the same.
Error 1452 (Cannot add or update a child row) happens on INSERT or UPDATE when the parent key is missing; insert the parent first or disable STRICT mode during migration.
Child table still contains rows that reference the parent key you want to delete.
The foreign key constraint was created without CASCADE or SET NULL, so MySQL blocks the delete.
Bulk scripts delete parent tables before child tables, violating referential integrity.
Data loads with foreign_key_checks turned off may create orphan rows, later triggering the error when deletes run.
Older MySQL versions return 1451 for the same foreign key violation.
Occurs on INSERT or UPDATE when the parent key is missing.
Appears if data violates referential integrity during load or migration.
Yes, but only temporarily and in controlled environments. Disabling checks can lead to data corruption if not reversed quickly.
The cascade adds minimal overhead. It is often faster than manual deletes because operations are executed within the same transaction.
The condition name was introduced in 5.7.4, but the underlying behavior remains in later versions with error code 1834.
Galaxy visualizes foreign key relationships, warns before dangerous deletes, and offers AI-generated refactors that respect referential integrity.
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