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.lidgroup 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
on75014, 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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