Re: Unique constraint and NULL values

From: DA Morgan <damorgan_at_x.washington.edu>
Date: Thu, 21 Oct 2004 23:47:10 -0700
Message-ID: <1098427577.727908@yasure>

Frank Piron wrote:

> Thu, 21 Oct 2004 18:54:29 -0700 DA Morgan <damorgan_at_x.washington.edu>
> wrote:
>

>> Two nulls should never be evaluated ... there is nothing to consider.
>> So considering them the same or different is irrelevant.
>>
>> CREATE TABLE t (
>> col1  NUMBER(1),
>> col2  VARCHAR2(1),
>> col3  DATE);
>>
>> INSERT INTO t (col1) VALUES (1);
>> COMMIT;
>>
>> What sense would it make to compare col2 and col3?
>> Would you consider them the same?

>
>
> No. I wanted to stress the fact that
> eval(NULL=NULL) "=" eval(NULL<>NULL) "=" "NULL"
> where the doublequotes indicate metalanguage.
>
> Unlike true and false, NULL acts on both levels
> because it can be a column's "value" and can be
> the result of a logical expression.

Demos of this can be found at http://www.psoug.org Click on Morgan's Library
Click on NULL (on the web 'NULL' has a value)

-- 
Daniel A. Morgan
University of Washington
damorgan_at_x.washington.edu
(replace 'x' with 'u' to respond)

Received on Fri Oct 22 2004 - 01:47:10 CDT