Половина из каждой группы

with t as (select 'Группа1' group_name from dual
           union all 
           select 'Группа2' group_name from dual
           union all 
           select 'Группа2' group_name from dual
           union all 
           select 'Группа3' group_name from dual
           union all 
           select 'Группа3' group_name from dual
           union all 
			  select 'Группа3' group_name from dual
           union all 
           select 'Группа3' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual)
select group_name
  from (select group_name,
               count(*) over(partition by group_name) cnt,
        			row_number() over(partition by group_name order by group_name) rnum
         from t) tt
 where cnt =1 or rnum <= cnt/2 

with t as (select 'Группа1' group_name from dual
           union all 
           select 'Группа2' group_name from dual
           union all 
           select 'Группа2' group_name from dual
           union all 
           select 'Группа3' group_name from dual
           union all 
           select 'Группа3' group_name from dual
           union all 
			  select 'Группа3' group_name from dual
           union all 
           select 'Группа3' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual
           union all 
           select 'Группа4' group_name from dual)
select rnum, group_name
  from (select group_name,
               count(*) over(partition by group_name) cnt,
        			row_number() over(partition by group_name order by group_name) rnum
         from t) tt
 where mod(rnum, 2) <> 0