3346
MySQLERRORCommonDDL / Foreign KeysHIGH confidence
Cannot change parent foreign key column
Production Risk
High — DDL fails; application may be left without required schema changes.
How to reproduce
trigger — this will error
trigger — this will error
ALTER TABLE customers MODIFY id BIGINT; -- id is referenced by orders.customer_id FK
expected output
ERROR 3346 (HY000): Cannot change column 'id' because it is used in a foreign key constraint.
Fix
Drop FKs on child, alter both, recreate
Drop FKs on child, alter both, recreate
ALTER TABLE orders DROP FOREIGN KEY fk_cust; ALTER TABLE customers MODIFY id BIGINT; ALTER TABLE orders MODIFY customer_id BIGINT; ALTER TABLE orders ADD CONSTRAINT fk_cust FOREIGN KEY (customer_id) REFERENCES customers(id);
Why this works
Consistent type changes across parent and child tables allow FKs to be restored.
What not to do
✕
Sources
Official documentation ↗
MySQL 8.0 — 3346 ER_FOREIGN_KEY_COLUMN_CANNOT_CHANGE
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