PostgreSQL version: 7.4.3 (RPMs from ftp.au.postgresql.org)

Operating Sysem: Fedora Core 1

CREATE TABLE t(i integer UNIQUE);
INSERT INTO t VALUES (null);
INSERT INTO t VALUES (null);
SELECT coalesce(i,-999) FROM t;
coalesce
----------
-999
-999
(2 rows)



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David Newall wrote:
NULL = NULL is not true, so the rows are not equal.


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On Tue, 6 Jul 2004, David Newall wrote:

NULL values are explicitly allowed to be duplicated in unique constraints
by spec AFAICS. The unique constraint is defined in terms of the unique
predicate which says: "If there are no two rows in T such that the value
of each column in one row is non-null and is equal to the value of the
corresponding column in the other row according to Subclause 8.2,
"<comparison predicate>", then the result of the <unique predicate> is
true; otherwise, the result of the <unique predicate> is false."



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David Newall wrote:

As someone says NULL = NULL is false
so that two rows are different. If you want enforce
only one null value for that column you have to write
you own trigger.



Regards
Gaetano Mendola



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On Mon, Jul 12, 2004 at 14:47:34 +0200,
Gaetano Mendola <mendola (AT) bigfoot (DOT) com> wrote:
As someone else pointed out, NULL = NULL is NULL which is not TRUE.

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Bruno Wolff III wrote:

Yes, that it's better.


Regards
Gaetano Mendola



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Gaetano Mendola wrote:
Still not precise.

NULL = NULL
is NULL (which is neither TRUE nor FALSE)

while
(NULL = NULL) IS NULL
is TRUE...


Regards,
Andreas

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