greatest() function returns undefined property

I think you want to use get_var(), not get_row()looking at the documentation, MySQL’s GREATEST just returns a single value.

$results will then just be the value of the greatest (be it up or down).

If you want the greatest up and the greatest down, use;

$wpdb->get_row("SELECT GREATEST(up) AS 'up', GREATEST(down) AS 'down' ...")