PL/SQL INSTEAD OF triggers - Oracle PL/SQL Tutorial

The Oracle PL/SQL INSTEAD OF trigger is a special type of trigger that is associated with a view. Unlike BEFORE or AFTER triggers, which are typically associated with tables, an INSTEAD OF trigger is specifically designed for views. This type of trigger allows you to define custom actions to be performed instead of the default insert, update, or delete operations on a view.

The primary purpose of an INSTEAD OF trigger is to provide a way to customize the behavior of DML (Data Manipulation Language) operations on a view. Views in Oracle are virtual tables that are based on the result of a SELECT query. By default, views are read-only, and you cannot perform insert, update, or delete operations directly on them if they involve multiple base tables or complex queries. However, by using an INSTEAD OF trigger, you can override this default behavior and define your own logic for handling modifications to the view.

Here’s a basic example to illustrate the concept of an INSTEAD OF trigger. Suppose you have a view named “EmployeeView” that combines information from the “Employees” and “Departments” tables. The view might look like this:

CREATE VIEW EmployeeView AS
SELECT e.employee_id, e.employee_name, e.department_id, d.department_name
FROM Employees e
JOIN Departments d ON e.department_id = d.department_id;

By default, you can’t directly perform an update on this view because it involves multiple tables. However, you can create an “INSTEAD OF” trigger to handle updates in a customized way:

CREATE OR REPLACE TRIGGER InsteadOfUpdateEmployeeView
INSTEAD OF UPDATE ON EmployeeView
FOR EACH ROW
BEGIN
  -- Custom logic for handling updates
  UPDATE Employees
  SET employee_name = :new.employee_name
  WHERE employee_id = :old.employee_id;

  -- Additional logic, if needed

  COMMIT;
END;
/

In this example, the trigger is fired instead of the default update operation on the “EmployeeView” view. It contains custom logic to update the corresponding row in the “Employees” table based on the changes made to the view.

It’s important to note that INSTEAD OF triggers can be used not only for updates but also for inserts and deletes on views. They provide a powerful mechanism for implementing complex business rules and enforcing data integrity when working with views that involve multiple tables. However, it’s crucial to use them judiciously, as improper implementation can lead to unexpected behavior and performance issues.

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