Unique and nulls ?

[Bart Pietercil]

Hi List,

is it possible to combine for a given field the nullable and unique  
properties in such a way that the null values are excluded from the  
unique test ?

Case: field mailaddress
This field should be unique but I do not always have a mailaddress so  
it needs to accept null values. These null values should be excluded  
from the unique test.

TIA

Bart Pietercil