• I found the example difficult to follow. To demonstrate the technique you could use one table with one WHERE without XML processing that is not easy for entry-level developers and professional level administrators and without joins. Also there is too much stress on NULL as a defult value. Do not forget that we mostly are in the described situation when users have to select something and they do it through the front end. The developers  often set default value for the dropdown boxes to something meaningful like "please, select the value from the list" or in many cases "All"

    Regards,Yelena Varsha