How Exceptions Are Raised
By the run-time system, the internal exceptions are raised implicitly as are user-defined exceptions that you have related with an Oracle error number using an EXCEPTION_INIT. Though, other user-defined exceptions should be raised explicitly by the RAISE statements.
Using the RAISE Statement
The PL/SQL subprograms and blocks must raise an exception only when an error makes it undesirable or impossible to finish the processing. You can place the RAISE statements for a given exception anywhere within the scope of that exception. In the illustration below, you alert your PL/SQL block to a user-defined exception named out_of_stock:
DECLARE
out_of_stock EXCEPTION;
number_on_hand NUMBER(4);
BEGIN
...
IF number_on_hand < 1 THEN
RAISE out_of_stock;
END IF;
EXCEPTION
WHEN out_of_stock THEN
-- handle the error
END;
You can also raise a predefined exception explicitly. In that way, an exception handler written for the predefined exception can process other errors, as the illustration below shows:
DECLARE
acct_type INTEGER;
BEGIN
...
IF acct_type NOT IN (1, 2, 3) THEN
RAISE INVALID_NUMBER; -- raise predefined exception
END IF;
EXCEPTION
WHEN INVALID_NUMBER THEN
ROLLBACK;
...
END;