Making a column immutable in MySQL I want to make a column in a relation unmodifyable for consistency reasons. 

The backstory is that I have a n:n relationship where the "connecting" relation has additional values. I don't want that anyone is able to change the IDs of the relationship partners. 

What I have come up with so far:

Creating a trigger that checks if the NEW value is the same as the OLD value. But I don't know how to deny the UPDATE action if the condition proves as TRUE. 

Here is my trigger so far:

CREATE TRIGGER deny_nton_change BEFORE UPDATE ON Schueler_in_Klasse
FOR EACH ROW BEGIN
    IF NEW.Schueler_ID != OLD.Schueler_ID OR NEW.Klasse_ID != OLD.Klasse_ID THEN

    END IF;
END;
Rather than abort the update, just override it?

...
FOR EACH ROW BEGIN
    SET NEW.Schueler_ID = OLD.Schueler_ID;
    SET NEW.Klasse_ID = OLD.Klasse_ID;
END