Oracle 11g: only return a value if row is first or last in group

You can try using the lag() and lead() analytical functions:

with a as (
   select 1 group_id, 100 count_1, 123 count_2 from dual union all
   select 1 group_id, 101 count_1, 123 count_2 from dual union all
   select 1 group_id, 283 count_1, 342 count_2 from dual union all
   select 1 group_id, 134 count_1, 123 count_2 from dual union all
   select 2 group_id, 241 count_1, 432 count_2 from dual union all
   select 2 group_id, 321 count_1, 920 count_2 from dual union all
   select 2 group_id, 432 count_1, 121 count_2 from dual union all
   select 2 group_id, 135 count_1, 342 count_2 from dual 
)
select
  case lag (group_id) over (order by group_id, count_1) 
    when group_id then 
      case lead (group_id) over (order by group_id, count_1)
      when group_id then null
           else 'last'
      end
    else to_char(group_id) end x,
  count_1,
  count_2
from
a;

with lag(group_id) over (order by group_id, count_1) you get the group_id of the previous record (it lags behind). Similarly, with lead(group_id) over... you get the group_id of the next record.

With the case expression, you compare the current group_id with the next and previous one and then return the appropriate value.

Not tested, but this should be the idea. If you need to sort by COUNT_1 or COUNT_2, you should include it in the analytic functions' over clause, partition by GROUP_ID order by COUNT_1

Refer here to understand what an analytic function is.

select
  case when ROW_NUMBER = 1 then GROUP_ID
       when ROW_NUMBER = GROUP_COUNT then 'last'
       else NULL
  end GROUP_ID
  ,COUNT_1
  ,COUNT_2
from(
    select
      GROUP_ID
      ,COUNT_1
      ,COUNT_2
      ,row_number() over(partition by GROUP_ID) ROWNUMBER
      ,count(GROUP_ID) over (partition by GROUP_ID) GROUP_COUNT
    from
      FOO
)

CREATE TABLE tt(g NUMBER, c1 NUMBER, c2 NUMBER);
INSERT INTO tt VALUES(1, 100, 123);
INSERT INTO tt VALUES(1, 101, 123);
INSERT INTO tt VALUES(1, 283, 342);
INSERT INTO tt VALUES(1, 134, 123);
INSERT INTO tt VALUES(2, 241, 432);
INSERT INTO tt VALUES(2, 321, 920);
INSERT INTO tt VALUES(2, 432, 121);
INSERT INTO tt VALUES(2, 135, 342);

SELECT CASE WHEN 1=ROW_NUMBER() OVER (PARTITION BY g ORDER BY c1 ASC,  c2 ASC) THEN '1'
            WHEN 1=ROW_NUMBER() OVER (PARTITION BY g ORDER BY c1 DESC, c2 DESC) THEN 'Last'
            ELSE 'Empty'
        END answer,
       c1, c2
  FROM tt;

1        100  123
Empty    101  123
Empty    134  123
Last     283  342
1        135  342
Empty    241  432
Empty    321  920
Last     432  121