SQL> SELECT ename,
  2         DECODE(deptno, 10, 'ACCOUNTING',
  3                        20, 'RESEARCH',
  4                        30, 'SALES',
  5                        40, 'OPERATIONS',
  6                            'UNKNOWN') AS department
  7    FROM emp
  8   WHERE rownum < 4
  9  /

ENAME      DEPARTMENT
---------- ----------
SMITH      RESEARCH
ALLEN      SALES
WARD       SALES

SQL> SELECT ename,
  2         CASE deptno
  3           WHEN 10 THEN 'ACCOUNTING'
  4           WHEN 20 THEN 'RESEARCH'
  5           WHEN 30 THEN 'SALES'
  6           WHEN 40 THEN 'OPERATIONS'
  7           ELSE
  8                        'UNKNOWN'
  9         END AS department
 10    FROM emp
 11   WHERE rownum < 4
 12  /

ENAME      DEPARTMENT
---------- ----------
SMITH      RESEARCH
ALLEN      SALES
WARD       SALES

SQL> select decode(NULL
  2               ,NULL,'NULL'
  3               ,'NOT NULL'
  4               ) x
  5    from dual
  6  /

X
----
NULL

SQL> SELECT CASE NULL
  2         WHEN NULL THEN
  3           'NULL'
  4         ELSE
  5           'ELSE'
  6         END x
  7    FROM dual
  8  /

X
----
ELSE

SQL> create or replace procedure pro_01(n number) is
  2  begin 
  3  dbms_output.put_line(' The number  = '||n);
  4  End;
  5  /

Procedure created.

SQL> set serverout on
SQL> var a varchar2(5);
SQL> Begin
  2  :a := 'ONE';
  3  End;
  4  /

PL/SQL procedure successfully completed.

SQL> Begin
  2   pro_01(Decode(:a,'ONE',1,0));
  3  End;
  4  /
 pro_01(Decode(:a,'ONE',1,0));
        *
ERROR at line 2:
ORA-06550: line 2, column 9:
PLS-00204: function or pseudo-column 'DECODE' may be used inside a SQL
statement only
ORA-06550: line 2, column 2:
PL/SQL: Statement ignored


SQL> Begin
  2    pro_01(case :a when 'ONE' then 1 else 0 end);
  3  End;
  4  /
The number  = 1

PL/SQL procedure successfully completed.

SQL>