Pragma Exception_INIT- Doubt

Question

Hi All
Why we are using Pragma Exception_INIT what is the purpose ?
Declare    
      e_MissingNull EXCEPTION;
     PRAGMA EXCEPTION_INIT(e_MissingNull, -1400);
BEGIN
     INSERT INTO Employees (employee_id) VALUES (NULL);
EXCEPTION
     WHEN e_MissingNull then
        DBMS_OUTPUT.put_line('ORA-1400 occurred');
END;when i give the above code i get
PL/SQL procedure successfully completed. ORA-1400 occurred
The same code when i try to give error message is ORA-1500 occurred
I get
ORA-01400: cannot insert NULL into ("XXI"."EMPLOYEES"."EMPLOYEE_ID")
ORA-06512: at line 6
why it happens?
And when i use try to get the error messgae to display Not valid instead of Zero_divide
Declare   
       I Number;
      e_sample EXCEPTION;
     PRAGMA EXCEPTION_INIT(e_sample, -5737);
BEGIN
     select 1/0 Into I From Dual ; -- I know Zero_Divide Error i thought the changing this error in my style
EXCEPTION
     WHEN e_smple then
        DBMS_OUTPUT.put_line('ORA-5737 Not valid');
END;But the Result is
ORA-01476: divisor is equal to zero
Please let me know the purpose of and usage for Pragma Exception_INIT
Thank in advance
Suresh

Answer 1

You must to issue:
SET SERVEROUTPUT ONto tell SQL*PLus to display DBMS_OUTPUT buffer:
SQL> Declare    
  2        e_MissingNull EXCEPTION;
  3       PRAGMA EXCEPTION_INIT(e_MissingNull, -1400);
  4  BEGIN
  5       INSERT INTO Employees (employee_id) VALUES (NULL);
  6  EXCEPTION
  7       WHEN e_MissingNull then
  8          DBMS_OUTPUT.put_line('ORA-1400 occurred');
  9  END;
10  /
PL/SQL procedure successfully completed.
SQL> SET SERVEROUTPUT ON
SQL> Declare    
  2        e_MissingNull EXCEPTION;
  3       PRAGMA EXCEPTION_INIT(e_MissingNull, -1400);
  4  BEGIN
  5       INSERT INTO Employees (employee_id) VALUES (NULL);
  6  EXCEPTION
  7       WHEN e_MissingNull then
  8          DBMS_OUTPUT.put_line('ORA-1400 occurred');
  9  END;
10  /
ORA-1400 occurred
PL/SQL procedure successfully completed.
SQL> Now about zero_divide. You are initializing exception e_sample with -5737, while zero_divide is -1476:
SQL> Declare
  2         I Number;
  3        e_sample EXCEPTION;
  4       PRAGMA EXCEPTION_INIT(e_sample, -1476);
  5  BEGIN
  6       select 1/0 Into I From Dual ; -- I know Zero_Divide Error i thought the changing this error in my style
  7  EXCEPTION
  8       WHEN e_sample then
  9          DBMS_OUTPUT.put_line('ORA-5737 Not valid');
10  END;
11  /
ORA-5737 Not valid
PL/SQL procedure successfully completed.
SQL> SY.