Re: Joins with nulls
Date: Fri, 22 Nov 2002 00:00:53 -0500
Message-ID: <GTiD9.8518$kS3.1257794_at_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 Fri Nov 22 2002 - 06:00:53 CET