An exception handler processes a raised exception (run-time error or warning condition). The exception can be either predefined or user-defined. Predefined exceptions are raised implicitly (automatically) by the run-time system. must be raised explicitly with either a
RAISE statement or the procedure
RAISE_APPLICATION_ERROR. The latter lets you associate an error message with the user-defined exception.
The name of either a predefined exception (such as
ZERO_DIVIDE), or a user-defined exception previously declared within the current scope.
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
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.
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 Table 11-1. 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.
Raise an exception 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.