SQL> select deptno, empno, ename from emp;

    DEPTNO      EMPNO ENAME
---------- ---------- ----------
        20       7369 SMITH
        30       7499 ALLEN
        30       7521 WARD
        20       7566 JONES
        30       7654 MARTIN
        30       7698 BLAKE
        10       7782 CLARK
        20       7788 SCOTT
        10       7839 KING
        30       7844 TURNER
        20       7876 ADAMS

    DEPTNO      EMPNO ENAME
---------- ---------- ----------
        30       7900 JAMES
        20       7902 FORD
        10       7934 MILLER

SQL> select empno, ename, deptno from emp where empno in (select min(empno) from emp group by deptno
);

     EMPNO ENAME          DEPTNO
---------- ---------- ----------
      7499 ALLEN              30
      7369 SMITH              20
      7782 CLARK              10

SEL eno, enam, dno
-> emp
? eno IN (SEL m(eno) 
          -> emp
          GBY dno
         );

SELECT MIN(employee_id) min_empid
     , department_id
FROM   employees
GROUP BY department_id
/

SQL> SELECT e.employee_id
  2       , e.first_name
  3  FROM   employees e
  4     ,   ( -- smallest employee_id
  5           SELECT MIN(employee_id) min_empid
  6                , department_id
  7           FROM   employees
  8           GROUP BY department_id
  9         ) s
 10  WHERE  s.min_empid   = e.employee_id
 11  /

EMPLOYEE_ID FIRST_NAME
----------- --------------------
        108 Nancy
        114 Den
        178 Kimberely
        201 Michael
        204 Hermann
        100 Steven
        205 Shelley
        120 Matthew
        203 Susan
        145 John
        200 Jennifer

EMPLOYEE_ID FIRST_NAME
----------- --------------------
        103 Alexander

12 rows selected.