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Let's say you run a single UPDATE statement that changes 5 rows in a table. Effect of the triggerĪ trigger can run either per row, or per statement. If you need to be sure the event actually is going to occur, AFTER is ideal. If you want to block an event like an INSERT, you will want to run BEFORE. Trigger BEFORE or AFTERĪ trigger can run either BEFORE or AFTER an event. If you don't include this list, updating any column will activate it. If UPDATE was one of the listed events, you can pass in a list of columns that should activate the trigger. Here are some examples of different events that can activate a trigger:Ī database trigger can also list more than one of these events. Here are the components to creating a trigger for your database:ĭatabase triggers will monitor tables for specific events.

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How to Create a SQL Trigger - Postgres Syntax They are as one, in perfect SQL matrimony. Since the event and the trigger function are all part of one atomic transaction, you know with absolute certainty that the trigger will fire if the event fires. Natural atomicity is another desirable feature bundled with triggers. If the logic is implemented on multiple application servers, you can no longer expect a clean, definitive change in behavior. The SQL server acts as a single point of truth. If you had a business requirement to multiply an incoming number by 10, and you wanted to revise this logic to multiply the number by 20, changing the logic in SQL would guarantee that every piece of data from that exact deploy time on would be affected by the new logic. Separating business logicĬoding critical business logic within the application code also presents problems when the business logic is updated.

Alternate solutions like Django's model hooks may fail if you have other application servers or users accessing the database who aren't aware of the specific business logic coded in your application. In some other engines like MySQL, the code block is a part of and inside the trigger.īefore I discuss what the different event types are and the specific syntax for creating a trigger, why would you want to use a database trigger? Advantages of using SQL Triggers Maintaining data integrityĭatabase triggers have a variety of uses and are an excellent tool to marshal strict data integrity. In Postgres, you delineate the code to run by creating a function whose return type is trigger. SQL Triggers, also called Database Triggers, allow you to tell your SQL engine (for these examples, Postgres) to run a piece of code when an event happens, or even before the event. Postgres Trigger Example 2: Creating an Audit Table.Postgres Trigger Example 1: Creating a Time Clock.This article contains all the information I wish I could have shared with my brother on how to effectively use SQL triggers. When building highly data oriented software, especially when the data is of financial nature and accuracy is of high demand, you're more likely to see data being manipulated at a lower level, in a more direct way. During my time working on custom ERP-like software, SQL triggers were an invaluable tool. In the world of open source start-up style full-stack development (think Django, Rails, Javascript, PHP, MySQL, Postgres.), ORMs are very popular and features like SQL triggers are far less conventional.īut there is still value with SQL triggers. With my only programming experience being recreating my favorite video game in VB6, I had little consolation to offer.įast forward 16 years, and now I can see from my brother’s perspective.

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Jonathan came home one day frustrated by a database full of convoluted SQL triggers.

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My brother, Jonathan, had just begun his software career at a startup. I was 12 years old when I first heard about SQL triggers.

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