Foreign Key Constraint Violation on Delete/Update

1. Deleting parent record with existing child records
2. Foreign key defined without CASCADE DELETE option
3. Updating primary key that's referenced elsewhere
4. Orphaned records from previous failed migrations
5. Manual foreign key in application instead of database

Check foreign key constraints in schema. If cascade delete is needed:

ALTER TABLE comments
ADD CONSTRAINT fk_user_id
FOREIGN KEY (user_id)
REFERENCES users(id)
ON DELETE CASCADE;

Or handle deletion order in application - delete children first, then parent.