Re: Functions in sort order - undocumented restriction

=?utf-8?q?PG_Doc_comments_form?= <noreply(at)postgresql(dot)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