QF: Group validation max 1 value

Hi! How can I validate a Group to have max. 1 value set? The manual shows how to get min. 1 I need that for a special field: in Databases I would like to have a select box followed by a text-field - like this: <Select a value> or set a <new> value The Selectbox keeps all the values in the Database (Select Distinct ...) and with <new> the user can set a custom value which will become a DB-value next time (by that it is easier to avoid different values that are actually same). For that reason I need a max 1 value group rule. Maybe someone could also invent a standard QF-field like that? If I would be more in QF I would do it ... Thanks Daniel