<p>The error appears when a foreign key operation would insert a duplicate value into the referenced table, violating referential integrity.</p>
<p>MySQL Error 1557 ER_FOREIGN_DUPLICATE_KEY_OLD_UNUSED arises when adding or updating a foreign key creates a duplicate key in the parent table. Remove or update the conflicting rows, then recreate the constraint to fix the issue.</p>
Upholding foreign key constraints for table '%s', entry
MySQL throws this error while upholding a foreign key constraint. The server detects that inserting, updating, or re-creating the foreign key would generate a duplicate key in the referenced table, breaking uniqueness rules. The process stops to protect referential integrity and prevent data corruption.
The error message typically reads: Upholding foreign key constraints for table 'child_table', entry 'value', key 1 would lead to a duplicate entry. It blocks the operation until the underlying conflict is solved.
The primary trigger is a pre-existing duplicate or orphaned record in either the parent or child table. When MySQL checks the foreign key, it discovers that the referenced key already exists with the same value, leading to a uniqueness clash.
Additional causes include legacy data imported without constraints, improper cascade rules, or partial deletes that left child rows pointing to nonexistent parents.
Locate the offending rows first. Use SELECT queries to find duplicate or orphaned values. Once identified, decide whether to delete, update, or merge records. After cleaning the data, recreate or enable the foreign key constraint.
Always run the fixes in a transaction and back up your tables. Validate results by re-running the foreign key creation statement. If no duplicates remain, the statement will succeed.
During schema migration, you might add a new foreign key to older data. Clean the dataset with DISTINCT queries or temporary staging tables before applying the constraint.
When importing CSV files, load them into a staging table, deduplicate records, then insert into the main tables with constraints enabled. This workflow prevents runtime errors.
Keep foreign key constraints enabled in development and staging to catch violations early. Use UNIQUE indexes on parent keys to guarantee single references.
Automate nightly integrity checks using INFORMATION_SCHEMA or mysqlcheck. Continuous monitoring highlights anomalies before they reach production.
MySQL Error 1452 (Cannot add or update a child row: a foreign key constraint fails) occurs when a child row references a non-existent parent rather than causing duplication. The fix involves inserting or correcting the missing parent record.
MySQL Error 1062 (Duplicate entry) surfaces when inserting a row that violates a UNIQUE index but is not tied to foreign keys. Resolve by removing or altering the duplicate value.
The parent table already contains two rows with the same primary or unique key, so attaching a foreign key exposes the duplication.
Child rows reference outdated parent IDs that were re-used, causing conflicts when the foreign key is enforced.
Data loaded in bulk bypassed constraint checks, leading to silent duplicates that appear only when the key is added later.
Manual deletes removed parent records but left conflicting child rows that now duplicate values during constraint creation.
Occurs when the child references a missing parent, not a duplicate. Add the parent or correct the reference.
Triggered by UNIQUE index violations without involving foreign keys. Remove or update the duplicate value.
Blocks deletion of a parent row that is still referenced by children. Delete or update child rows first.
SET FOREIGN_KEY_CHECKS = 0 lets the operation proceed, but it risks silent data corruption. Re-enable checks immediately and clean up duplicates before relying on constraints.
Error code 1557 exists in MySQL 5.7 and later, including MariaDB. The wording may change slightly, but the root cause and fix remain the same.
Galaxy's schema-aware autocomplete warns about missing or duplicate keys while you write SQL. Team members can endorse deduplication queries, ensuring everyone runs the trusted fix.
No. Analyze business logic first. Sometimes you should merge records or update foreign keys rather than blindly delete data.
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