3405
MySQLERRORNotableForeign KeysHIGH confidence
Cannot change foreign key child column (alternate path)
Production Risk
Low — DDL fails; no data is modified.
How to reproduce
trigger — this will error
trigger — this will error
ALTER TABLE child MODIFY COLUMN fk_col BIGINT;
expected output
ERROR 3405 (HY000): Cannot change column used in a foreign key constraint.
Fix
Drop FK, alter column, recreate FK
Drop FK, alter column, recreate FK
ALTER TABLE child DROP FOREIGN KEY fk_name; ALTER TABLE child MODIFY COLUMN fk_col BIGINT; ALTER TABLE child ADD FOREIGN KEY (fk_col) REFERENCES parent(id);
Why this works
Dropping the constraint before altering allows the column type change, then the constraint is recreated.
What not to do
✕
Sources
Official documentation ↗
MySQL 8.0 — 3405 ER_FK_COLUMN_CANNOT_CHANGE_CHILD2
Content generated with AI assistance and reviewed for accuracy. Found an error? hello@errcodes.dev