SQL> col empnos format a40
SQL> with 
  2    data as (
  3      select deptno, empno,
  4             lag(empno) over(partition by deptno order by empno) prev_emp
  5      from emp
  6    )
  7  select deptno,
  8         substr(sys_connect_by_path(empno, ','), 2) empnos
  9  from data
 10  where connect_by_isleaf = 1
 11  connect by prior empno = prev_emp
 12  start with prev_emp is null
 13  order by deptno
 14  /
    DEPTNO EMPNOS
---------- ----------------------------------------
        10 7782,7839,7934
        20 7369,7566,7788,7876,7902
        30 7499,7521,7654,7698,7844,7900