From: Gary Stainburn <gary.stainburn@ringways.co.uk>
Organization: Ringways Garages Ltd
To: pgsql-sql@postgresql.org
Subject: Re: left outer join only select newest record
Date: Wed, 23 May 2012 11:33:21 +0100
User-Agent: KMail/1.9.10
References: <201205231027.39198.gary.stainburn@ringways.co.uk>
	<CAFj8pRBAR5gNpuQAsCaQmGP=qy2JNqcJ-ZUPrWRUmE99WfNjhg@mail.gmail.com>
In-Reply-To: 
 <CAFj8pRBAR5gNpuQAsCaQmGP=qy2JNqcJ-ZUPrWRUmE99WfNjhg@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain;
  charset="utf-8"
Content-Transfer-Encoding: 7bit
Content-Disposition: inline
Message-Id: <201205231133.21123.gary.stainburn@ringways.co.uk>

On Wednesday 23 May 2012 10:46:02 Pavel Stehule wrote:
> select distinct on (s.s_registration) *
>  ... order by u.ud_id desc

I tried doing this but it complained about the order by.

goole=# select distinct on (s.s_stock_no) s_stock_no, s_regno, s_vin, 
s_created, ud_id, ud_handover_date from stock s left outer join used_diary u 
on s.s_regno = u.ud_pex_registration where s_stock_no = 'UL15470' order by 
s_stock_no, ud_id desc;
ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY expressions
goole=# select distinct on (s.s_stock_no) s_stock_no, s_regno, s_vin, 
s_created, ud_id, ud_handover_date from stock s left outer join used_diary u 
on s.s_regno = u.ud_pex_registration where s_stock_no = 'UL15470' order by  
ud_id desc;
ERROR:  SELECT DISTINCT ON expressions must match initial ORDER BY expressions
goole=# 

>
> or
>
> select *
>   from stock_details s
>           left join (select * from used_diary where (ud_id,
> ud_registration) = (select max(ud_id), ud_registration from used_diary
> group by ud_registration)) x
>           on s.s_registration = x.ud_registration;
>

This was more like what I was thinking, but I still get an error, which I 
don't understand.  I have extracted the inner sub-select and it does only 
return one record per registration. (The extra criteria is just to ignore old 
or cancelled tax requests and doesn't affect the query)

goole=# select distinct on (s.s_stock_no) s_stock_no, s_regno, s_vin, 
s_created, ud_id, ud_handover_date from stock s left outer join (select 
ud_id, ud_pex_registration, ud_handover_date from used_diary where (ud_id, 
ud_pex_registration) = (select max(ud_id), ud_pex_registration from 
used_diary where (ud_tab is null or ud_tab <> 999) and ud_created > 
CURRENT_DATE-'4 months'::interval group by ud_pex_registration)) udIn on 
s.s_regno = udIn.ud_pex_registration;
ERROR:  more than one row returned by a subquery used as an expression