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From: Ron Johnson <[email protected]>
To: pgsql-general <[email protected]>
Subject: Re: How do I check for NULL
Date: Mon, 8 Dec 2025 21:51:55 -0500
Message-ID: <CANzqJaAx6RYPzEpgHhioHm0xCQ=OhZjbs7aNHJD-Dex3LyfcpA@mail.gmail.com> (raw)
In-Reply-To: <CA+FnnTw=18Rba2XgceRUFKQo5P7Xf9QhXT6wKF=DgzYT6K9-sw@mail.gmail.com>
References: <CA+FnnTw=18Rba2XgceRUFKQo5P7Xf9QhXT6wKF=DgzYT6K9-sw@mail.gmail.com>

On Mon, Dec 8, 2025 at 9:40 PM Igor Korot <[email protected]> wrote:

> Hi, ALL,
> Consider the following scenario:
>
> CREATE TABLE test(a INT, b VARCHAR(256), c INT, d VARCHAR(256), /*
> more fields follows*/);
> CREATE UNIQUE INDEX test_x( b, c, d );
>
> Now I try to do:
>
> INSERT INTO test VALUES( 0, 'abc', 12345, (SELECT foo FROM bar),
> /*more data follow*/);
>
> My problem is:
>
> The SELECT can either return data or NULL.
> Everything is good when the data is returned, but the insert fails
> when the NULL is returned, because the field "d" is a part of UNIQUE
> INDEX.
>
> However,, I'd like to still insert the record and I'd like to do something
> like:
>
> INSERT INTO test VALUES( 0, 'abc', 12345, IF( (SELECT foo FROM bar) ==
> NULL, "postgres", <select_result>), /*more data follow*/);
>
> What would be the best way to achieve this?
>

https://www.postgresql.org/docs/15/sql-createindex.html section on NULLS
DISTINCT says
"Specifies whether for a unique index, null values should be considered
distinct (not equal). *The default* is that they are *distinct*, so
that a *unique
index could contain multiple null values in a column*."

That seems to mean multiple rows can have NULL in column "d".

-- 
Death to <Redacted>, and butter sauce.
Don't boil me, I'm still alive.
<Redacted> lobster!

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