Re: How to find Brothers and Sisters?

--CELKO-- wrote:

>>> , I do not recall that Codd's original specification of 1NF allowed NULL. <<

Those rules were not 'from the start'; they aren't in the 1970 paper, and were introduced in the 1980s, IIRC (certainly not earlier than the late 1970s).

> 3. Systematic treatment of null values
>
> The DBMS is required to support a representation of "missing
> information and inapplicable information" that is systematic, distinct
> from all regular values (for example, "distinct from zero or any other
> number," in the case of numeric values), and independent of data type.
> It is also implied that such representations must be manipulated by the
> DBMS in a systematic way.

-- 
Jonathan Leffler                   #include <disclaimer.h>
Email: jleffler_at_earthlink.net, jleffler_at_us.ibm.com
Guardian of DBD::Informix v2005.02 -- http://dbi.perl.org/