Error Handling in Triggers
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Errors and exceptions that occur in triggers may be handled using the WHEN statement. If an exception is handled with WHEN, the exception does not return a message to the application and does not necessarily terminate the trigger.
Error handling in triggers works the same as for stored procedures: the actions performed in the blocks up to the error-handling (WHEN) statement are undone and the statements specified by the WHEN statement are performed.
For more information on error handling with WHEN, see Working with Stored Procedures.