What are triggers in a relational database?

Triggers in a relational database are automated procedures that execute in response to specific events on a table, such as INSERT, UPDATE, or DELETE operations. They are defined to run either before or after the event occurs, allowing developers to enforce business rules, maintain data integrity, or automate tasks without requiring manual intervention. For example, a trigger could log changes to a user’s email address in an audit table every time an UPDATE operation occurs on the Users table. Triggers operate within the database layer, ensuring consistency even when multiple applications interact with the data.

Developers use triggers to handle tasks that should always follow or precede specific data modifications. Common use cases include auditing (tracking historical data changes), enforcing complex constraints beyond standard CHECK clauses, or synchronizing related tables. For instance, a trigger could automatically update a “last_modified” timestamp column after any row change. While triggers reduce the need for application-side logic, they can introduce performance overhead if overused or poorly designed. It’s important to ensure triggers are efficient and avoid cascading triggers (triggers that activate other triggers), which can complicate debugging and slow down operations.

Here’s a concrete example: suppose an Orders table has a trigger that fires AFTER INSERT. This trigger could calculate the total sales for a customer and update a CustomerSummary table. In SQL, this might look like:

CREATE TRIGGER UpdateCustomerSummary 
AFTER INSERT ON Orders 
FOR EACH ROW 
BEGIN 
 UPDATE CustomerSummary 
 SET total_spent = total_spent + NEW.amount 
 WHERE customer_id = NEW.customer_id; 
END;

Another example is a BEFORE INSERT trigger that validates an email format using a regular expression. Triggers are database-specific in syntax (e.g., PostgreSQL uses CREATE OR REPLACE TRIGGER, while MySQL requires DELIMITER adjustments). Properly designed triggers streamline workflows but require careful testing to avoid unintended side effects.