Your data is not really hierarchical, because there are no column(s) which provide for the perceived ordering you think is there. Your actual requirement is to sort by DNO group, and to then display the DNO value only once, for the “first” entry. We can approximate what you want using ROW_NUMBER():

WITH cte AS (
    SELECT e.*, ROW_NUMBER() OVER (PARTITION BY DNO ORDER BY DBMS_RANDOM.VALUE) rn
    FROM EMP e
)

SELECT CASE WHEN rn = 1 THEN DNO END AS DNO, ENAME, SAL
FROM cte t
ORDER BY t.DNO, rn;

Demo

If you have some logic by which we know which employee to report first for a given block of DNO, then you should add these details (and columns) to the question.

If you are using SQL*Plus (or another client application that supports SQL*Plus commands) then you can use BREAK:

BREAK ON dno
  
SELECT * FROM emp ORDER BY dno, ename;

If you are not using a client application that supports BREAK then you can use the ROW_NUMBER analytic function to number the rows and then a CASE expression and TO_CHAR to format the number as a string or a space character for later rows in each group:

SELECT CASE rn
       WHEN 1
       THEN TO_CHAR(dno)
       ELSE ' '
       END AS dno,
       ename,
       sal
FROM   (
         SELECT dno,
                ename,
                sal,
                ROW_NUMBER() OVER (PARTITION BY dno ORDER BY ename) AS rn
         FROM   emp
       ) e
ORDER BY
       e.dno,
       e.ename

Which outputs:

fiddle