Can we raise an exception within an exception? - oracle

Question

Can we raise an exception within an exception, like RAISE my_exception within an WHEN OTHERS exception?
Additionally, I am trying to understand the output in the following 2 scenarios:
(Note that the only difference is the ordering of the exceptions)
Scenario 1
DECLARE
lc_exception Exception;
var1 number;
BEGIN
select 1
into var1
from dual
where 1=2;
EXCEPTION
when lc_exception then
dbms_output.put_line('This is my exception');
when no_data_found then
raise lc_exception;
when others then
dbms_output.put_line('There could be some other exception');
END;
Scenario 2
DECLARE
lc_exception Exception;
var1 number;
BEGIN
select 1
into var1
from dual
where 1=2;
EXCEPTION
when no_data_found then
raise lc_exception;
when lc_exception then
dbms_output.put_line('This is my exception');
when others then
dbms_output.put_line('There could be some other exception');
END;

Answer 1

Both scenarios have the similar error stack:
ORA-06510: PL/SQL: unhandled user-defined exception
ORA-06512: at line 20
ORA-01403: no data found
The only difference is the exact location (line 20 vs. 18) where the user defined exception is raised.
The select matches no rows thus no_data_found is raised by Oracle - ORA-01403: no data found in the error stack.
The exception is caught by the block's exception handler block.
In the exception handler an user defined exception lc_exception is raised.
Because the program has no other exception handlers the user defined exception is leaked to host environment (in my case that was sqlplus client) - ORA-06510: PL/SQL: unhandled user-defined exception and program terminates.
The exception propagation is similar in both scenarios because in both cases the same initial exception no_data_found is raised and there is only one exception handling block. Therefore the order of no_data_found and lc_exception handlers doesn't matter.
More detailed explanation about exception handling/propagation is found from: PL/SQL Error Handling in Oracle documentation.

Answer 2

You just need an exception handler to catch your local exception. If you raise an exception in an exception handler, it cannot be caught in the same exception hander.
DECLARE
lc_exception EXCEPTION;
var1 number;
BEGIN
BEGIN
SELECT 1
INTO var1
FROM dual
WHERE 1=2;
EXCEPTION -- this is where the exception will be raised
WHEN no_data_found THEN
raise lc_exception;
END;
dbms_output.put_line( 'This will never be executed.' );
EXCEPTION -- this is where the exception will be caught
WHEN lc_exception THEN
dbms_output.put_line( 'This is my exception' );
WHEN others THEN
dbms_output.put_line( 'There could be some other exception' );
END;
However, you may want to consider a slightly different method.
DECLARE
lc_exception EXCEPTION;
PRAGMA EXCEPTION_INIT( lc_exception, -20222 );
var1 NUMBER;
BEGIN
BEGIN
SELECT 1
INTO var1
FROM dual
WHERE 1=2;
EXCEPTION
WHEN NO_DATA_FOUND THEN
RAISE_APPLICATION_ERROR( -20222, 'This is my exception' );
END;
DBMS_OUTPUT.PUT_LINE( 'This will never be executed.' );
EXCEPTION
WHEN lc_exception THEN
DBMS_OUTPUT.PUT_LINE( SQLERRM );
WHEN OTHERS THEN
DBMS_OUTPUT.PUT_LINE( 'Some other exception: ' || SQLERRM );
END;