From: Tom Lane <tgl@sss.pgh.pa.us>
To: andrew@grillet.co.uk
cc: pgsql-docs@lists.postgresql.org
Subject: Re: Functions in sort order - undocumented restriction
In-reply-to: <151826283029.1443.7699948679319699778@wrigleys.postgresql.org>
References: <151826283029.1443.7699948679319699778@wrigleys.postgresql.org>
Comments: In-reply-to =?utf-8?q?PG_Doc_comments_form?=
 <noreply@postgresql.org>
	message dated "Sat, 10 Feb 2018 11:40:30 +0000"
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Date: Sat, 10 Feb 2018 13:56:37 -0500
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=?utf-8?q?PG_Doc_comments_form?= <noreply@postgresql.org> writes:
> This does not work:

>  select distinct (o.bid), organisation, posttown, replace(case when postcode
> ='' then null else trim(postcode) end, ' ', '') as pc, phone, o.active,
> website, email, (select count(*) from notes as n where n.bid = o.bid) as nn
> from organisations as o right join notes as n on o.bid = n.bid where true
> order by replace(case when phone ='' then null else trim(phone) end, ' ',
> '') nulls last ;
> ERROR:  for SELECT DISTINCT, ORDER BY expressions must appear in select list

What do you find unclear about this error message?

The reason seems clear enough to me: if the ordering expression isn't one
of the values that are being de-duplicated on, then there isn't a unique
value to associate with each surviving row for sorting purposes.

			regards, tom lane