with t as (
select 'G1' grp from dual union all
select 'G1' from dual union all
select 'G1' from dual union all
select 'G2' from dual union all
select 'G2' from dual union all
select 'G2' from dual
)
select grp
  from (
         select grp
              , dense_rank()over(order by grp) rk
           from t
       )
 where rk = 1