PL/SQL User's Guide and Reference

How Exceptions Are Raised

Internal exceptions are raised implicitly by the runtime system, as are user-defined exceptions that you have associated with an Oracle error number using EXCEPTION_INIT. However, other user-defined exceptions must be raised explicitly by RAISE statements.

Using the RAISE Statement

PL/SQL blocks and subprograms should raise an exception only when an error makes it undesirable or impossible to finish processing. You can place RAISE statements for a given exception anywhere within the scope of that exception. In the following example, 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. That way, an exception handler written for the predefined exception can process other errors, as the following example 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;