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Learn how to create a trigger in PostgreSQL using an existing function to automatically update order amounts and ensure data integrity in your database.
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Creating a Trigger in PostgreSQL to Update Order Amounts Using an Existing Function

Introduction

PostgreSQL is a powerful, open-source relational database management system that offers a rich feature set including advanced data types, extensibility, and sophisticated locking mechanisms. One of the many powerful features in PostgreSQL is the ability to create triggers. Triggers allow you to automatically execute a specified function in response to certain events on a particular table, such as insertions, updates, or deletions.

In this post, we will walk you through how to create a trigger in PostgreSQL that utilizes an existing function to update order amounts automatically.

Prerequisites

Before we start, make sure you have the following:

A PostgreSQL database instance up and running.

Basic knowledge of SQL and PL/pgSQL.

An existing table and function that you want to use with the trigger.

What is a Trigger?

A trigger is a special type of stored procedure that automatically runs when certain events occur in the database:

BEFORE (before the change is applied)

AFTER (after the change is applied)

INSTEAD OF (in place of the triggering operation)

Triggers can be activated by three types of events:

INSERT

UPDATE

DELETE

Creating a Trigger Function

First, you need an existing function that calculates the order amount. Suppose we already have a function named calculate_order_amount. Below is an example implementation:

[[See Video to Reveal this Text or Code Snippet]]

This function will calculate the order_amount based on the quantity and unit_price of an order before the row is inserted into the table.

Defining the Trigger

Next, we need to define the trigger that will call this function. We create the trigger using the CREATE TRIGGER statement:

[[See Video to Reveal this Text or Code Snippet]]

In this command:

BEFORE INSERT OR UPDATE specifies that the trigger will fire before the INSERT or UPDATE operations.

ON orders specifies the target table.

FOR EACH ROW means the trigger will execute once for each row affected by the triggering event.

EXECUTE FUNCTION calculate_order_amount() specifies the function to be executed.

Putting It All Together

To summarize, here is the complete SQL script that defines the function and the trigger:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By following these steps, you can create a trigger in PostgreSQL that will use an existing function to automatically update order amounts. This not only ensures data integrity but also simplifies your SQL operations by automating the necessary calculations.

Triggers are a powerful tool for managing and maintaining your database, ensuring that certain actions are consistently and automatically performed whenever data is modified.