Use of raise in exception handling block

Question

what is the use of raise in exception handling block for eg.
declare
a number;
b emp.empno%type;
begin
begin
SELECT empno INTO a FROM emp where 1=2;
exception
when others then
dbms_output.put_line('inner');
raise;
end;
exception
when no_data_found then
dbms_output.put_line('outer');
end;
output will be like below ..
inner
outer
PL/SQL procedure successfully completed.
my question is wht is the use of using raise in exception handing part, is there any specific reason we use in the development ????
Regards,
AAK.

Answer 1

In the first block, you do not raise you user-defined exception WHEN_NO_DATA_FOUND, but the predefined one, which is raised to the WHEN OTHERS exception handler.
Consider:
SQL> declare
  2     a number;
  3     my_err exception;
  4     no_data_found exception;
  5  begin
  6     begin
  7        select 1 into a from dual where 1=2;
  8     exception
  9     when no_data_found then
10        dbms_output.put_line(' In system defined');
11        raise my_err;
12     end;
13  exception
14     when my_err then
15        dbms_output.put_line('In User Defined');
16     when others then
17        declare
18           v_sqlerrm varchar2(100);
19        begin
20           v_sqlerrm := sqlerrm;
21        dbms_output.put_line(' In when others '||sqlerrm);
22        end;
23  end;
24  /
In when others ORA-01403: no data found
PL/SQL procedure successfully completed.
SQL> ed
Wrote file afiedt.buf
  1  declare
  2     a number;
  3     my_err exception;
  4     --no_data_found exception;
  5  begin
  6     begin
  7        select 1 into a from dual where 1=2;
  8     exception
  9     when no_data_found then
10        dbms_output.put_line(' In system defined');
11        raise my_err;
12     end;
13  exception
14     when my_err then
15        dbms_output.put_line('In User Defined');
16     when others then
17        declare
18           v_sqlerrm varchar2(100);
19        begin
20           v_sqlerrm := sqlerrm;
21        dbms_output.put_line(' In when others '||sqlerrm);
22        end;
23* end;
SQL> /
In system defined
In User Defined
PL/SQL procedure successfully completed.
SQL>Note that in the second block, I deleted the declaration of the user defined exception NO_DATA_FOUND.
This is taken from the documentation:
Redeclaring Predefined Exceptions
Remember, PL/SQL declares predefined exceptions globally in package STANDARD, so you need not declare them yourself. Redeclaring predefined exceptions is error prone because your local declaration overrides the global declaration. For example, if you declare an exception named invalid_number and then PL/SQL raises the predefined exception INVALID_NUMBER internally, a handler written for INVALID_NUMBER will not catch the internal exception. In such cases, you must use dot notation to specify the predefined exception, as follows:
EXCEPTION
  WHEN invalid_number OR STANDARD.INVALID_NUMBER THEN
    -- handle the error
END;You can read yourself :
http://download-uk.oracle.com/docs/cd/B19306_01/appdev.102/b14261/errors.htm
Regards,
Gerd