Error
Error Code:
1832
MariaDB Error 1832: Cannot Alter Foreign Key Column
Description
This error indicates that you attempted to modify a table column (e.g., using `ALTER TABLE ... CHANGE COLUMN` or `MODIFY COLUMN`) that is currently part of an active foreign key constraint. MariaDB prevents such changes to ensure data consistency and prevent orphaned records in related tables, as altering the column would invalidate the foreign key relationship.
Error Message
Cannot change column '%s': used in a foreign key constraint '%s'
Known Causes
4 known causesColumn Data Type Mismatch
You attempted to alter the data type of a column that is either a foreign key or part of a primary key referenced by a foreign key, making it incompatible with the referencing column.
Renaming Referenced Column
The error occurs when you try to rename a column that is actively involved in a foreign key constraint, which would break the established relationship.
Dropping Referenced Column
You tried to drop a column that is an integral part of a foreign key constraint, which is disallowed to maintain referential integrity.
Modifying Column Attributes
Changing attributes like NULLability, character set, or length of a column linked by a foreign key can lead to this error if the modification invalidates the constraint.
Solutions
3 solutions available1. Drop and Recreate Foreign Key medium
Temporarily remove the foreign key constraint, alter the column, and then re-add the constraint.
1
Identify the table and column involved in the foreign key constraint. The error message will usually provide these details.
2
Find the name of the foreign key constraint. You can do this by querying the `INFORMATION_SCHEMA.KEY_COLUMN_USAGE` table.
SELECT CONSTRAINT_NAME, TABLE_NAME, COLUMN_NAME FROM INFORMATION_SCHEMA.KEY_COLUMN_USAGE WHERE REFERENCED_TABLE_NAME = 'your_parent_table' AND REFERENCED_COLUMN_NAME = 'your_parent_column';3
Drop the foreign key constraint. Replace `your_child_table` and `your_fk_constraint_name` with the actual names.
ALTER TABLE your_child_table DROP FOREIGN KEY your_fk_constraint_name;4
Alter the column in the child table. Replace `your_child_table`, `your_column`, and the desired new definition.
ALTER TABLE your_child_table MODIFY COLUMN your_column new_column_definition;5
Re-add the foreign key constraint with the modified column. Ensure the `ON UPDATE` and `ON DELETE` clauses match the original constraint if they existed.
ALTER TABLE your_child_table ADD CONSTRAINT your_fk_constraint_name FOREIGN KEY (your_column) REFERENCES your_parent_table(your_parent_column) ON DELETE CASCADE ON UPDATE CASCADE;2. Use a Temporary Table for Alterations advanced
Create a new table with the desired schema, copy data, drop the old table, and rename the new one.
1
Create a new temporary table with the desired column definition. Replace `your_child_table`, `your_new_child_table`, and `new_column_definition`.
CREATE TABLE your_new_child_table LIKE your_child_table;2
Modify the column definition in the new temporary table.
ALTER TABLE your_new_child_table MODIFY COLUMN your_column new_column_definition;3
Copy all data from the original table to the new temporary table. Ensure all columns are listed.
INSERT INTO your_new_child_table (col1, col2, your_column, ...) SELECT col1, col2, your_column, ... FROM your_child_table;4
Drop the original foreign key constraint from the original table. You might need to find the constraint name first as in Solution 1.
ALTER TABLE your_child_table DROP FOREIGN KEY your_fk_constraint_name;5
Drop the original table.
DROP TABLE your_child_table;6
Rename the new temporary table to the original table name. This will also recreate any primary keys and indexes.
RENAME TABLE your_new_child_table TO your_child_table;7
Re-add the foreign key constraint to the now-renamed table. Ensure the `ON UPDATE` and `ON DELETE` clauses match the original constraint if they existed.
ALTER TABLE your_child_table ADD CONSTRAINT your_fk_constraint_name FOREIGN KEY (your_column) REFERENCES your_parent_table(your_parent_column) ON DELETE CASCADE ON UPDATE CASCADE;3. Disable Foreign Key Checks Temporarily easy
Temporarily disable foreign key checks to perform the alteration, then re-enable them.
1
Disable foreign key checks for the current session.
SET foreign_key_checks = 0;2
Alter the column in the child table. Replace `your_child_table`, `your_column`, and the desired new definition.
ALTER TABLE your_child_table MODIFY COLUMN your_column new_column_definition;3
Re-enable foreign key checks for the current session.
SET foreign_key_checks = 1;4
Verify that the foreign key constraint is still valid. If the data types are incompatible, this might still cause issues or require further data manipulation.