Martijn,

>> M> As far as I can remember, IB and Fb1 did allow one NULL into an
>> M> unique INDEX, but not in a constraint. I might be wrong there though.
>>
>> no, not even one.

M> Did a quick test ... which I should have done before, of course.

<SNIP>

what you have found is documented SQL and Firebird 1.5 behaviour, AFAIK.
You did test with Fb 1.5, I hope!
With previous versions you should get an error here:

M> CREATE UNIQUE ASC INDEX I_IND_TEST ON INDEX_TEST (C1);

M> Maybe it has something to do with many engines using indices to implement
M> constraints.

Sure it has. The motto "keys aren't indexes and indexes aren't keys"
is there in my toolbox right beside "null is a state, not a value".
:-)

Ciao
--
Nando mailto:nandod@...