SET serveroutput ON
DECLARE
  l_deptno INTEGER DEFAULT 10;
  v_dname  VARCHAR2(200);
BEGIN
  l_deptno:='&deptno';
  SELECT dname INTO v_dname FROM dept WHERE deptno=l_deptno;
  dbms_output.put_line('dname value='||v_dname);
EXCEPTION
WHEN no_data_found THEN
  dbms_output.put_line('error');
END;
/

SQL> @test.sql
Enter value for deptno: 40
dname value=OPERATIONS

SQL> @test.sql
Enter value for deptno:
error

dname value=ACCOUNTING

DECLARE
V_DEFAULT NUMBER := 5;
V_COUNT NUMBER;
BEGIN
SELECT COUNT(*) INTO V_COUNT FROM DUAL CONNECT BY LEVEL <= DECODE('&&INPUT1','',V_DEFAULT,'&&INPUT1');
DBMS_OUTPUT.PUT_LINE(V_COUNT);
SELECT COUNT(*) INTO V_COUNT FROM DUAL CONNECT BY LEVEL <= DECODE('&&INPUT2','',V_DEFAULT,'&&INPUT2');
DBMS_OUTPUT.PUT_LINE(V_COUNT);
END;
/

SQL> DECLARE
  2    l_deptno INTEGER DEFAULT 10;
  3    v_dname  VARCHAR2(200);
  4  BEGIN
  5    l_deptno:='&deptno';
  6    SELECT dname INTO v_dname FROM dept WHERE deptno=nvl(l_deptno,10);
  7    dbms_output.put_line('dname value='||v_dname);
  8  EXCEPTION
  9  WHEN no_data_found THEN
 10    dbms_output.put_line('error');
 11  END;
 12  /
Enter value for deptno: 40
dname value=OPERATIONS

PL/SQL procedure successfully completed.

SQL> /
Enter value for deptno: 
dname value=ACCOUNTING

PL/SQL procedure successfully completed.

l_deptno:='&deptno';

l_deptno:='';