Oracle PL/SQL Programming: A Developer's Workbook

Chapter 4. Exception Handling

Unlike politicians, who can resort to tired phrases like “Mistakes were made,” PL/SQL developers must make sure their programs behave responsibly in the face of the unpredictable. For example, a database server can run out of memory; a user can attempt to insert a duplicate value for a primary key; a SELECT…INTO clause can return too many rows. You can use exception handlers to trap, or handle, these exceptions.

There are two steps to handling an exception:

1. Define the conditions that represent exceptions; you can supplement the extensive Oracle-supplied set (for example, DUP_VAL_ON_INDEX) by creating exceptions of your own (for example, PROFIT_TOO_LOW), by associating unnamed exceptions with your own names (via the EXCEPTION_INIT pragma), or even by defining your own unnamed exceptions.

2. Create an exception handling section in your code, where you associate a subset of named exceptions with corresponding blocks of code called handlers.

When an exception occurs (whether it’s generated by the system or you use the RAISE command to create it), program control immediately branches to the handler associated with that exception. If there is no handler for that particular exception (or no exception section at all), you have an unhandled exception, in which case the program terminates immediately and returns the error to the original caller. This chapter tests your ability to define your own named exceptions, create an exception section in your code, and understand ...