unable to call exception block in oracle - oracle

Question

I have trying to execute below pl sql block in my oracle developer edition.I have made calls to functions and procedures and it works fines.But i am not able to call exception in case my query does not get executed.I have been trying to get a wrong query exectued by passing a string value instead of int value.So it throws error but also i need to get exception block to executed in case of such error.Block 2 should through exception as i am passing string value.But exception block does not get call,Any help?? Below is my block
DECLARE
DBCID INT := 102;
CNT INT;
BEGIN
SELECT DEVOPS_ISDBCEXECUTED(DBCID, 'DDL') INTO CNT FROM DUAL;
IF (CNT = 0) THEN
BEGIN
DEVOPS_DBCINSERT (DBCID,'DDL','hsolanki','Prj1','Item1','avarne');
BEGIN
DECLARE W_CNT int;
BEGIN
SELECT COUNT(*) INTO W_CNT FROM HS WHERE NAM = 'DK'; //block 1
IF (W_CNT = 0) THEN
INSERT INTO HS
(NAM, AGE)
VALUES ('Dk',8);
END IF;
END;
END;
BEGIN
DECLARE W_CNT int;
BEGIN
SELECT COUNT(*) INTO W_CNT FROM HS WHERE NAM = 'Ab';
IF (W_CNT = 0) THEN
INSERT INTO HS
(NAM, AGE) //block 2
VALUES ('Ab',s);
END IF;
END;
END;
DEVOPS_DBCUPDATE(DBCID, NULL,'SUCCESS');
EXCEPTION
WHEN OTHERS THEN
DBMS_OUTPUT.PUT_LINE ('ERROR OCCURED : ' || sqlerrm);
DEVOPS_DBCUPDATE (DBCID,sqlerrm,'Failed');
rollback;
END;
END IF;
END;

Answer 1

Your exception handling block is within the 'IF (CNT = 0) THEN .. END IF' block. If you pass a string value, most probably the exception was thrown at the first function call ( SELECT DEVOPS_ISDBCEXECUTED...), which is not 'protected' by an excpetion handler. You would need to move the exception handler to the outermost block, e.g.:
DECLARE
DBCID INT := 102;
CNT INT;
BEGIN
....
EXCEPTION
WHEN OTHERS THEN
...
END;

Answer 2

So we know ...
DEVOPS_DBCUPDATE is a procedure which updates a table
... and ...
IN exception i am calling DEVOPS_DBCUPDATE ... my table is not getting updated
... and ...
i dont know what is pragma autonomous_transaction
Putting these clues altogether we can see that the rollback in the EXCEPTION block will wipe out the change to the table executed by the preceding call to DEVOPS_DBCUPDATE(), so it only seems as though the EXCEPTION block is not being executed ( a check on whether the DBMS_OUTPUT message is displayed would confirm that it is being called).
Anyway the solution is to make DEVOPS_DBCUPDATE() run in a nested transaction, so the change is applied regardless of what happens in the wider transaction. We do this with the autonomous_transaction pragma.
Obviously I don't know the exact structure of your code, but it will look something like this:
create or replace procedure DEVOPS_DBCUPDATE( ... ) is
pragma autonomous_transaction;
begin
update your_table
set ....
commit;
end;
The COMMIT in the procedure will persist the change to your table but will not save anything in the outer transaction. So the rollback in the EXCEPTION block would still reverse the inserts into the HS table.
Autonomous transactions are very useful when employed properly, but it is easy to misuse them. This scenario - persistent logging in the event of exception or rollback - is their main use case. But generally, use with caution: it's easy to abuse autonomous transactions and end up with a corrupted database. There's more information in the Oracle documentation.

Answer 3

An error raise in the DECLARE section is not handled by that block's EXCEPTION section. For this reason it is often safer to initialise variables after the BEGIN i.e.
DECLARE
DBCID INT;
CNT INT;
BEGIN
DBCID := 'xxx';
...
EXCEPTION
WHEN OTHERS THEN
... -- The assignment will be caught here
END;