sub-sel/group problem

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
=#
--
Gary Stainburn

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