create trigger keine_degradierung
before update on uni.Professoren
for each row
when (old.Rang is not null)
begin
        if :old.Rang = 'C3' and :new.Rang = 'C2' then
                :new.Rang := 'C3';
        end if;
        if :old.Rang = 'C4' then
                :new.Rang := 'C4';
        end if;
        if :new.Rang is null then
                :new.Rang := :old.Rang;
        end if;
end;