Using FIRST_VALUE in triggers

20 Oct 2023

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Triggers in databases allow developers to automatically execute code when certain events occur, such as updates, inserts, or deletes on a table. One common use case for triggers is to perform calculations or update values based on changes in the database.

In this blog post, we will explore how to use the FIRST_VALUE function in triggers. FIRST_VALUE is a window function that retrieves the first value in an ordered set of values within a specific partition. It can be useful in triggers to capture the initial value of a column before it is modified.

Creating a Trigger

To demonstrate the usage of FIRST_VALUE in triggers, let’s consider a scenario where we have a “orders” table with columns order_id, order_amount, and order_status. We want to track the initial status of an order whenever it is modified.

To achieve this, we can create a trigger that captures the first value of order_status using FIRST_VALUE and inserts it into a separate “order_history” table.

Here’s an example of how to create such a trigger in SQL:

CREATE TRIGGER capture_order_status
AFTER UPDATE ON orders
FOR EACH ROW
WHEN (OLD.order_status <> NEW.order_status)
BEGIN
    INSERT INTO order_history (order_id, initial_status)
    VALUES (NEW.order_id, FIRST_VALUE(OLD.order_status) OVER (PARTITION BY NEW.order_id ORDER BY order_id));
END;

In the above example, the trigger is fired after an update occurs on the “orders” table. It checks if the order_status has changed (OLD.order_status <> NEW.order_status). If there is a change, it inserts the order_id and the first value of order_status into the “order_history” table using the FIRST_VALUE function.

Testing the Trigger

To test the trigger, you can execute update statements on the “orders” table and observe the changes in the “order_history” table. For example:

UPDATE orders
SET order_status = 'Cancelled'
WHERE order_id = 123;

This update statement will trigger the capture_order_status trigger, and the initial status of the order will be stored in the “order_history” table.

Conclusion

Using the FIRST_VALUE function in triggers allows developers to capture the initial value of a column before it is modified. This can be helpful in scenarios where you need to keep a history of changes in your database. By using triggers and window functions like FIRST_VALUE, you can automate the process of capturing such changes and storing them in separate tables.

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