CREATE OR REPLACE TRIGGER users_city_change
    AFTER UPDATE ON users_city
    FOR EACH ROW

DECLARE
    old_id NUMBER;
    new_id NUMBER;

BEGIN
    old_id := :old.users_id;
    new_id := :NEW.users_id;

    IF (old_id = new_id) THEN
        INSERT INTO users_city_change_log(
            users_id,
            old_city_id,
            new_city_id
        ) VALUES (
            old_id,
            :old.city_id,
            :NEW.city_id
        );
    END IF;
END users_city_change;