Re: sub-sel/group problem

On Mon, 11 Aug 2003, Gary Stainburn wrote:

> 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;

I assume concat is an aggregate function?

>
> 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

Something like this might work:
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 rtid
order by rtid;

IIRC, SQL99 has a complicated way of defining dependant columns for group
by but SQL92 only allows you to use the columns that are grouped outside
of an aggregate. In general, if r.rlid is >0 in some cases and <0 in
others for a particular group, what would the query do? You might be able
to show that it doesn't happen, but the system isn't quite that bright. ;)