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_at_yasure>
>
>
> 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.
Date: Thu, 21 Oct 2004 23:47:10 -0700
Message-ID: <1098427577.727908_at_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 - 08:47:10 CEST