Exceptions

An exception is a runtime error or warning condition, which can be predefined or user-defined. Predefined exceptions are raised implicitly (automatically) by the runtime system. User-defined exceptions must be raised explicitly by RAISE statements. To handle raised exceptions, you write separate routines called exception handlers. For more information, see Chapter 10.

Syntax

Description of exception_declaration.gif follows

Description of the illustration exception_declaration.gif

Keyword and Parameter Description

exception_name

A predefined exception such as ZERO_DIVIDE, or a user-defined exception previously declared within the current scope.

OTHERS

Stands for all the exceptions not explicitly named in the exception-handling part of the block. The use of OTHERS is optional and is allowed only as the last exception handler. You cannot include OTHERS in a list of exceptions following the keyword WHEN.

statement

An executable statement. For the syntax of statement, see "Blocks".

WHEN

Introduces an exception handler. You can have multiple exceptions execute the same sequence of statements by following the keyword WHEN with a list of the exceptions, separating them by the keyword OR. If any exception in the list is raised, the associated statements are executed.

Usage Notes

An exception declaration can appear only in the declarative part of a block, subprogram, or package. The scope rules for exceptions and variables are the same. But, unlike variables, exceptions cannot be passed as parameters to subprograms.

Some exceptions are predefined by PL/SQL. For a list of these exceptions, see "Summary of Predefined PL/SQL Exceptions". PL/SQL declares predefined exceptions globally in package STANDARD, so you need not declare them yourself.

Redeclaring predefined exceptions is error prone because your local declaration overrides the global declaration. In such cases, you must use dot notation to specify the predefined exception, as follows:

EXCEPTION
   WHEN invalid_number OR STANDARD.INVALID_NUMBER THEN ...

The exception-handling part of a PL/SQL block is optional. Exception handlers must come at the end of the block. They are introduced by the keyword EXCEPTION. The exception-handling part of the block is terminated by the same keyword END that terminates the entire block. An exception handler can reference only those variables that the current block can reference.

An exception should be raised only when an error occurs that makes it undesirable or impossible to continue processing. If there is no exception handler in the current block for a raised exception, the exception propagates according to the following rules:

If there is an enclosing block for the current block, the exception is passed on to that block. The enclosing block then becomes the current block. If a handler for the raised exception is not found, the process repeats.
If there is no enclosing block for the current block, an unhandled exception error is passed back to the host environment.

Only one exception at a time can be active in the exception-handling part of a block. Therefore, if an exception is raised inside a handler, the block that encloses the current block is the first block searched to find a handler for the newly raised exception. From there on, the exception propagates normally.

Example

The following PL/SQL block has two exception handlers:

DECLARE
   bad_emp_id  EXCEPTION;
   bad_acct_no EXCEPTION;
   ...
BEGIN
   ...
EXCEPTION
   WHEN bad_emp_id OR bad_acct_no THEN  -- user-defined
      ROLLBACK;
   WHEN ZERO_DIVIDE THEN  -- predefined
      INSERT INTO inventory VALUES (part_number, quantity);
      COMMIT;
END;