Error
Error Code:
1762
MySQL Error 1762: Foreign Key Duplicate Child Entry
Description
This error indicates that an operation on a parent table would violate a unique constraint in a child table via a foreign key relationship. Specifically, an update or insert on the parent table's referenced key would cause a foreign key value in the child table to become a duplicate, which is not allowed due to a unique index.
Error Message
Foreign key constraint for table '%s', record '%s' would lead to a duplicate entry in a child table
Known Causes
3 known causesParent Key Update Conflict
An update to the primary or unique key in the parent table attempts to set it to a value that already exists in the child table's foreign key column, violating a unique constraint.
Incorrect Child Table Unique Constraint
The child table has a unique index on its foreign key column(s), and the parent operation tries to introduce a foreign key value that would violate this unique constraint, often indicating a design issue.
Cascading Action Duplication
An `ON UPDATE CASCADE` or `ON DELETE CASCADE` action on the foreign key is triggered by a parent operation, leading to duplicate foreign key values in the child table where a unique constraint exists.
Solutions
4 solutions available1. Identify and Remove Duplicate Child Records medium
Find and delete duplicate rows in the child table that violate the foreign key constraint.
1
Identify the specific duplicate child record(s) causing the error. The error message often provides clues about the table and record. You might need to query the child table based on the columns involved in the foreign key constraint.
SELECT column1, column2, COUNT(*) FROM child_table WHERE foreign_key_column = 'value_from_error_message' GROUP BY column1, column2 HAVING COUNT(*) > 1;2
Once duplicates are identified, determine which record to keep and which to delete. This often involves examining other columns for uniqueness or based on business logic.
3
Delete the duplicate child record(s). Be very careful with this step as deleting the wrong record can lead to data loss.
DELETE FROM child_table WHERE id_column = duplicate_record_id;4
Retry the operation that initially caused the error.
2. Modify the Parent Record to Avoid Duplication easy
Adjust the data in the parent table record being inserted or updated to prevent a duplicate child entry.
1
Examine the parent table record that is being inserted or updated. Understand which column(s) in the parent table are being used to establish the foreign key relationship in the child table.
2
If the parent record's value for the foreign key column (or a combination of columns that form the unique key in the child table) already exists in the child table in a way that would create a duplicate, you need to modify the parent record.
3
Update the parent record with a value that does not conflict with existing child records, or ensure the combination of values being inserted into the child table via the parent operation is unique.
UPDATE parent_table SET foreign_key_column = 'new_unique_value' WHERE id = parent_record_id;4
Retry the operation that initially caused the error.
3. Temporarily Disable and Re-enable Foreign Key Checks easy
A quick workaround for bulk operations, but use with caution to avoid data integrity issues.
1
Disable foreign key checks for the current session.
SET foreign_key_checks = 0;2
Perform the operation that was causing the error (e.g., bulk insert, update).
INSERT INTO child_table (...) VALUES (...);3
Re-enable foreign key checks.
SET foreign_key_checks = 1;4
After re-enabling, it's crucial to verify data integrity. You may still need to address underlying duplicate data if it was not intended.
4. Review and Adjust Table Schema (if applicable) advanced
Ensure the child table's unique constraints align with the foreign key's intent.
1
Examine the `CREATE TABLE` statements for both the parent and child tables, paying close attention to the foreign key definition and any `UNIQUE` constraints on the child table's columns that are part of the foreign key relationship.
SHOW CREATE TABLE child_table;2
Determine if the `UNIQUE` constraint on the child table is correctly defined to prevent the exact duplicate entries that the foreign key constraint is trying to enforce. Sometimes, the unique constraint might be missing or incorrect.
3
If the unique constraint is missing or incorrect, add or alter it. This might involve adding a new `UNIQUE` index or modifying an existing one.
ALTER TABLE child_table ADD CONSTRAINT uc_fk_columns UNIQUE (fk_column1, fk_column2);4
If the existing unique constraint is causing unintended duplicates (meaning the business logic allows for multiple identical entries under certain circumstances), you might need to re-evaluate the data model or the foreign key relationship itself.