SQL> SELECT e.last_name
  2       , e.department_id
  3       , CASE
  4            WHEN department_id = 100
  5               THEN NULL
  6            WHEN department_id = 60
  7               THEN (SELECT l.city
  8                     FROM   locations l
  9                          , departments d
 10                     WHERE  d.department_id = e.department_id
 11                     AND    l.location_id = d.location_id)
 12            WHEN department_id = 90
 13               THEN (SELECT department_name
 14                     FROM   departments
 15                     WHERE  department_id = e.department_id)
 16         END mytest
 17  FROM   employees e
 18  WHERE  ROWNUM < 11
 19  /

LAST_NAME                 DEPARTMENT_ID MYTEST
------------------------- ------------- ------------------------------
King                                 90 Executive
Kochhar                              90 Executive
De Haan                              90 Executive
Hunold                               60 Southlake
Ernst                                60 Southlake
Austin                               60 Southlake
Pataballa                            60 Southlake
Lorentz                              60 Southlake
Greenberg                           100
Faviet                              100

10 rows selected.

SQL> SELECT e.ename
  2       , e.deptno
  3       , CASE
  4            WHEN deptno = 10
  5               THEN NULL
  6            WHEN deptno = 30
  7               THEN (SELECT d.loc
  8                     FROM   dept d
  9                     WHERE  d.deptno = e.deptno)
 10            WHEN deptno = 20
 11               THEN (SELECT d.dname
 12                     FROM   dept d
 13                     WHERE  d.deptno = e.deptno)
 14         END mytest
 15  FROM   emp e
 16  WHERE  ROWNUM < 11
 17  /

ENAME          DEPTNO MYTEST
---------- ---------- --------------
KING               10
BLAKE              30 CHICAGO
CLARK              10
JONES              20 RESEARCH
MARTIN             30 CHICAGO
ALLEN              30 CHICAGO
TURNER             30 CHICAGO
JAMES              30 CHICAGO
WARD               30 CHICAGO
FORD               20 RESEARCH

10 rows selected.