Parameter Problem

I have two parameter - Action and Result.Action - available value from a query - SELECT actionid, actionvalue from ACTIONResult - available value from a query - SELECT resultid , resultvalue from ACTIONRESULT where actionid = actionid from Action parameterHowerer not all actionid in ACTIONRESULT, now the users want if the actionid not in ACTIONRESULT, display all result from the ACTIONRESULT table.There is no expression in the available value in parameter so I don't know how to make it to work.Any idea??