Message-ID: <3F379ED5.3030408@klaster.net>
Date: Mon, 11 Aug 2003 15:49:09 +0200
From: Tomasz Myrta <jasiek@klaster.net>
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To: Gary Stainburn <gary.stainburn@ringways.co.uk>
Cc: PostgreSQL-sql <pgsql-sql@postgresql.org>
Subject: Re: sub-sel/group problem
References: <200308111124.07832.gary.stainburn@ringways.co.uk>
In-Reply-To: <200308111124.07832.gary.stainburn@ringways.co.uk>
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> Hi folks,
> 
> I don;t know if it's cos it's Monday or what, but I can't see what's wrong 
> here.
> 
> I've got two tables, turns which holds a turn number, a task, and where 
> appropriate a link to a loco.  The select below works but only shows those 
> tasks where a loco is involved.:
> 
> select r.rtid, concat(r.rcid::text || ' on ' || l.lnumber::text) as task 
>     from rides r, loco_dets l where r.rlid = l.lid group by rtid;
> 
>  rtid |                                     task                                     
> ------+---------------------------------------------
>     5 | G on 60007
>     6 | A on 75014, C on 75014, A on 75029, C on 75029
>     7 | C on 4277, A on 44767, C on 44767
>     8 | A on 30926, C on 30926, G on 60532
>     9 | A on 30926, C on 30926, A on 75014, C on 75014
>    10 | F on 2392, F on 75029, L on 75029
>    11 | A on 44767, C on 44767, A on 75029
> 
> However, when I tried to change this to using an outer join I'm getting stuck. 
> Can anyone see my stumbling point, which I think is to do with the condition 
> part of the case statement.  Do I need to do that in a sub-select first or is 
> there an alternative?
> 
> =# select r.rtid,
> -#      case when r.rlid > 0 then
> -#        concat(r.rcid::text || ' on ' || l.lnumber::text)
> -#      else
> -#        r.rcid::text
> -#      end as task
> -#     from rides r
> -#      left outer join loco_dets l on r.rlid = l.lid
> -#     group by rtid
> -#     order by rtid
> -# ;
> ERROR:  Attribute r.rlid must be GROUPed or used in an aggregate function
> =#
Watch out.
When using outer joins you get NULL values.

some_text || NULL = NULL (always)

Try this:

select r.rtid,
  concat(r.rcid::text || coalesce(' on ' || l.lnumber::text,'')
  as task
from rides r
left outer join loco_dets l on r.rlid = l.lid
group by r.rtid
order by r.rtid

Regards,
Tomasz Myrta