On Thu, 20 Jan 2005, Dann Corbit wrote:
> It is clear to me that only allowing a single null value will not
> violate the explanation below.
Given two rows in T with one column each
(NULL), (NULL)
Find two rows such that the value of each column in one row is non-null
and equal to the value of the corresponding column in the other row
according to 8.2. If there are no such rows the unique predicate returns
true (ie the constraint is satisfied).