Redeclaring Predefined Exceptions

Keep in mind that, the PL/SQL declares predefined exceptions globally in the package STANDARD; Therefore you need not declare them yourself. Re-declaring the predefined exceptions is error prone as your local declaration overrides the global declaration. For illustration, if you declare an exception named invalid_number and then PL/SQL raises the predefined exception INVALID_NUMBER internally, a handler written for the INVALID_ NUMBER will not catch the internal exception. In such situations, you should use dot notation to specify the predefined exception, which is as shown:

EXCEPTION

WHEN invalid_number OR STANDARD.INVALID_NUMBER THEN

-- handle the error

END;