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Description

This error indicates that you are attempting to create a foreign key constraint using a name that is already in use within the current database schema. MySQL requires all constraint names to be unique to properly identify and manage database objects, preventing the new constraint from being created.

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Error Message

Duplicate foreign key constraint name '%s'

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Known Causes

4 known causes

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Re-executing DDL Script

You might be running a SQL script to create tables and foreign keys multiple times without first dropping the existing tables or constraints, leading to an attempt to re-create an already existing foreign key constraint name.

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Typographical Error or Copy-Paste

Accidentally copying and pasting a foreign key definition within the same script or across different DDL statements can lead to the same constraint name being used multiple times.

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Inconsistent Naming Convention

Without a clear and consistent naming convention for foreign key constraints, it's easy to inadvertently assign the same name to different constraints across various tables or even within the same table if not properly checked.

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Collaborative Schema Development

In a team environment where multiple developers are modifying the same database schema, a foreign key constraint might be added with a name already chosen by another developer.

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Solutions

3 solutions available

1. Rename the Duplicate Foreign Key Constraint easy

Manually rename one of the foreign key constraints to a unique name.

1

Identify the table and the duplicate foreign key name. The error message usually provides this information.

2

Connect to your MySQL server using a client like `mysql` or MySQL Workbench.

3

Drop the foreign key constraint with the duplicate name from one of the tables. Replace `your_table_name`, `your_foreign_key_name`, and `your_database_name` with your actual values.

ALTER TABLE your_table_name DROP FOREIGN KEY your_foreign_key_name;
USE your_database_name;
SHOW CREATE TABLE your_table_name;

4

Add the foreign key constraint back with a unique name. Replace `your_table_name`, `your_new_foreign_key_name`, `column_name`, `referenced_table_name`, and `referenced_column_name` with your actual values.

ALTER TABLE your_table_name ADD CONSTRAINT your_new_foreign_key_name FOREIGN KEY (column_name) REFERENCES referenced_table_name(referenced_column_name);

2. Remove Redundant Foreign Key Definition medium

Remove the duplicate foreign key definition from the SQL script or schema.

1

Examine the SQL script or the `CREATE TABLE` statements that are causing this error. Look for two or more `CONSTRAINT` clauses that define a foreign key with the same name and on the same table (or related tables where the name collision occurs).

2

Identify the specific foreign key constraint that is being defined twice with the same name. The error message `Duplicate foreign key constraint name '%s'` will provide the name.

3

Delete or comment out one of the duplicate `CONSTRAINT` definitions from your SQL script. Ensure that you retain only one definition for each unique foreign key.

-- Example of a duplicate foreign key definition to remove:
-- ALTER TABLE your_table_name ADD CONSTRAINT fk_user_id FOREIGN KEY (user_id) REFERENCES users(id);

ALTER TABLE your_table_name ADD CONSTRAINT fk_user_id FOREIGN KEY (user_id) REFERENCES users(id);

4

Re-run the corrected SQL script.

3. Inspect and Clean Existing Schema advanced

Check the database schema for existing duplicate foreign key names and remove them.

1

Connect to your MySQL server.

2

Query the `information_schema.KEY_COLUMN_USAGE` table to find foreign keys and their names. This query will list all foreign key constraints and their associated table and constraint names.

SELECT CONSTRAINT_NAME, TABLE_NAME, REFERENCED_TABLE_NAME FROM information_schema.KEY_COLUMN_USAGE WHERE REFERENTIAL_CONSTRAINT_NAME IS NOT NULL AND TABLE_SCHEMA = 'your_database_name' ORDER BY CONSTRAINT_NAME, TABLE_NAME;

3

Analyze the output of the query. Look for instances where the same `CONSTRAINT_NAME` appears multiple times for different tables or even the same table (though less common for FKs).

4

For each identified duplicate foreign key name, decide which one to keep and which one to drop. You might need to `SHOW CREATE TABLE` for the involved tables to understand the context.

SHOW CREATE TABLE your_table_name;

5

Drop the redundant foreign key constraint. Replace `your_table_name` and `your_foreign_key_name` with your actual values.

ALTER TABLE your_table_name DROP FOREIGN KEY your_foreign_key_name;

6

Re-attempt the operation that initially caused the error (e.g., applying migrations, creating tables).

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Related Errors

5 related errors

MySQL Error 1826: Duplicate Foreign Key Name