BEFORE UPDATE Trigger FAQS

1. What is a BEFORE UPDATE trigger in Oracle?

A BEFORE UPDATE trigger in Oracle is automatically invoked before an UPDATE operation is applied to a table. It allows you to modify values, validate data, enforce business rules, and prevent updates under certain conditions before the data is actually changed.

2. When does the BEFORE UPDATE trigger execute?

The BEFORE UPDATE trigger is fired before the update operation is executed on a row in the table. This gives you the chance to:

• Modify the values that will be updated.
• Perform validation checks on the data.
• Prevent the update if certain conditions are not met.

3. Can I modify the data in a BEFORE UPDATE trigger?

Yes, in a BEFORE UPDATE trigger, you can modify the new values using the :NEW pseudo-record before they are written to the table. This allows you to adjust data or apply default values before it gets committed.

Example:

:NEW.salary := :NEW.salary * 1.1;  -- Automatically increase salary by 10%

4. What are the :NEW and :OLD pseudo-records?

• :NEW: Refers to the new values that are being updated or inserted into the table.
• :OLD: Refers to the old values (prior to the update) of the row being updated.

In a BEFORE UPDATE trigger:

• You can access the old values through :OLD to compare or validate them against the new values in :NEW.
• You can also modify :NEW values before they are saved to the table.

5. Can I prevent the update from happening in a BEFORE UPDATE trigger?

Yes, you can prevent the update by raising an exception in the trigger. If you raise an exception, the update operation will be canceled and no changes will be made to the row.

Example:

IF :NEW.salary < :OLD.salary THEN

  RAISE_APPLICATION_ERROR(-20001, 'Salary cannot be decreased.');

END IF;

6. What is the difference between a row-level and statement-level trigger?

• Row-level trigger: Fires once for each row being updated. It allows you to access both the old and new values for each individual row and is specified using FOR EACH ROW.
• Statement-level trigger: Fires once for the entire UPDATE statement, regardless of how many rows are affected. It doesn't allow access to individual row data.

A BEFORE UPDATE trigger is typically defined as a row-level trigger, allowing actions to be performed on each row.

7. Can I use a BEFORE UPDATE trigger to modify multiple rows?

Yes, if the UPDATE statement modifies multiple rows, a row-level trigger will execute once for each row affected. However, if you're using a statement-level trigger, it will execute only once for the entire statement, regardless of the number of rows updated.

8. How do I log changes made in a BEFORE UPDATE trigger?

You can use a BEFORE UPDATE trigger to log changes to a separate audit table. Typically, you will log both the old and new values by referencing :OLD and :NEW.

Example:

CREATE OR REPLACE TRIGGER log_employee_update

BEFORE UPDATE

ON employees

FOR EACH ROW

BEGIN

  INSERT INTO employee_audit_log (emp_id, old_salary, new_salary, update_time)

  VALUES (:OLD.employee_id, :OLD.salary, :NEW.salary, SYSDATE);

END;

9. Can a BEFORE UPDATE trigger be fired by an INSERT or DELETE operation?

No, a BEFORE UPDATE trigger is specifically designed to fire when an UPDATE operation is executed on the table. It will not fire for INSERT or DELETE operations.

10. What happens if an error occurs in a BEFORE UPDATE trigger?

If an error occurs in a BEFORE UPDATE trigger (e.g., raising an exception or encountering a runtime error), the entire update operation will be rolled back. This includes both the trigger and the actual update statement.

To handle errors, you can use exception handling within the trigger:

BEGIN

  -- Trigger logic

EXCEPTION

  WHEN OTHERS THEN

    -- Handle error

    RAISE;  -- Optionally re-raise the error

END;

11. Can I have multiple BEFORE UPDATE triggers on the same table?

Yes, you can have multiple BEFORE UPDATE triggers on the same table, but the order of execution is not guaranteed unless explicitly controlled. You can manage execution order by using trigger PRIORITY or by ensuring that triggers don't conflict with each other.

12. Can I call a stored procedure in a BEFORE UPDATE trigger?

Yes, you can call a stored procedure from within a BEFORE UPDATE trigger. This is useful for encapsulating logic that might be reused across multiple triggers or tables.

Example:

CREATE OR REPLACE TRIGGER validate_employee_update

BEFORE UPDATE

ON employees

FOR EACH ROW

BEGIN

  validate_salary(:NEW.salary);  -- Call stored procedure

END;

 

13. How do I disable a BEFORE UPDATE trigger temporarily?

To disable a trigger temporarily, use the following SQL statement:

ALTER TRIGGER trigger_name DISABLE;

To enable it again:

ALTER TRIGGER trigger_name ENABLE;

14. Can I create a BEFORE UPDATE trigger on a view?

Yes, you can create a BEFORE UPDATE trigger on a view, but it will only work if the view is updatable. The view must map directly to the underlying base table(s) without any complex operations like aggregation, grouping, or joins that would prevent updating.

15. What are some common use cases for BEFORE UPDATE triggers?

• Data Validation: Ensuring that updates meet specific criteria (e.g., preventing salary reductions, checking for null values).
• Data Modification: Automatically adjusting values before they are committed (e.g., setting default values for NULL fields).
• Preventing Invalid Updates: Blocking updates under certain conditions (e.g., preventing updates to inactive records).
• Audit Logging: Keeping track of changes to critical data by logging old and new values in an audit table.

16. What are the performance implications of using a BEFORE UPDATE trigger?

• Triggers add some overhead to DML operations. A BEFORE UPDATE trigger will cause the update operation to take longer, especially if the trigger contains complex logic or involves multiple rows.
• It's important to ensure that the trigger logic is optimized for performance, especially when handling bulk updates or large tables.

17. How can I optimize the performance of a BEFORE UPDATE trigger?

• Keep the logic simple: Avoid complex queries, joins, or large computations within the trigger.
• Limit the number of operations: Only perform essential operations in the trigger.
• Use bulk processing: For large-scale updates, consider using a batch process or bulk collect to minimize context switching.