1451: Cannot delete or update a parent row: a foreign key constraint fails

What does it mean?

You tried to delete a parent row or change its key, but the foreign key constraint does not allow it because dependent child rows exist.

Why does it happen?

The referenced parent row still has child rows and the constraint uses RESTRICT/NO ACTION for ON DELETE/UPDATE.

How to fix it

First delete the dependent child rows or move them to another parent, then delete the parent row. If you want automatic behavior, redefine the constraint with ON DELETE CASCADE or ON DELETE SET NULL. Find dependent rows: SELECT * FROM child_table WHERE fk_column = value.