Re: Joins with nulls

From: Finarfin <finarfin_at_sympatico.ca>
Date: Fri, 22 Nov 2002 00:00:53 -0500
Message-ID: <GTiD9.8518$kS3.1257794@news20.bellglobal.com>

David Cressey wrote:

>> Nulls are not values, are a kind of mark.

>
> Add to what you said, that nulls are themselves the result of outer
> joins on the underlying data.
>
> A relational model of the data will not have any nulls in it.

I am curious about the above point. I was under the impression that one of Codd's original rules was that a relation should be allowed to have an empty field. As such, any system that failed to have a systematic approach to Nulls fails to be fully relational. Certainly when I studied set theory, we discussed the Null set. I've been trying to come up with a table design that wouldn't have a potential for Null. For instance:
Name:
First Name, Last Name,
Bono.

OR
Address:
Street:
No fixed address

OR
Groceries:
Number of cans of cat food:
Dog owner

OR
Supplier:
S#, Name, Status, City:
S1 Smith 10 No city, bankrupt

From this, it seems that the only table design that guarentees no nulls is (at most) 2 columns. First is an ID and the second is the particular attribute.

John Eggert

Note to Joe C. No DDL posted as these examples are independant of implementation. Received on Thu Nov 21 2002 - 23:00:53 CST