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From: Steve Midgley <[email protected]>
To: Shaozhong SHI <[email protected]>
Cc: pgsql-sql <[email protected]>
Subject: Re: A function to find errors in groups in a table
Date: Thu, 9 Jun 2022 12:16:12 -0700
Message-ID: <CAJexoSJ3g9W1XtdUVnss+5cw=r-ek-JSfiGUaBxCGG=BJLmHQQ@mail.gmail.com> (raw)
In-Reply-To: <CA+i5JwZCt0c1V-LexLo50Oa+r_UVLM26ZQ4pqq2d9XYaBebKjA@mail.gmail.com>
References: <CA+i5JwZ1y=KNn8uJr_DLe2LZCeLxVxY0osHBVoaELUVC=JWr6A@mail.gmail.com>
	<CA+i5JwZCt0c1V-LexLo50Oa+r_UVLM26ZQ4pqq2d9XYaBebKjA@mail.gmail.com>

On Thu, Jun 9, 2022 at 6:37 AM Shaozhong SHI <[email protected]> wrote:

> There is a table full of grouped values like the following
>
> nodeid     link_type      primary
> 11           outflowlink       1
> 11              inflowlink      1
> 11              outflowlink     2
>
> Primary of 1 indicates a primary water course.  Primary of 2 indicates a
> secondary water course.
>
> Obviously, one of the out flow links is an error, as its primacy value is
> 2.  It is wrong that water flows from a primary water course into a
> secondary water course.
>
> How can a function can be designed to find and report such errors?
>

Is the problem that there exists this row:

11              outflowlink     2

But there is no corresponding row:

11              inflowlink     2

So that you need to find all "orphan" rows that don't have a corresponding
member with the opposing data field inflowlink or outflowlink where the
"primary" column is the join field between the two rows?

Thanks for clarifying your problem,
Steve

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