Triggers in SQL are special types of stored procedures that automatically execute in response to specific database events, such as INSERT, UPDATE, DELETE, or DDL operations. They are tied directly to a table or view and run when the defined event occurs, either before or after the operation. For example, a trigger could log changes to a table every time a row is updated or enforce data validation rules before a record is inserted. Triggers are useful for automating repetitive tasks, maintaining data integrity, or synchronizing data across tables without requiring manual intervention from developers.
Triggers can be categorized based on when they execute (BEFORE or AFTER an event) and their scope (row-level or statement-level). A BEFORE trigger runs prior to the event, allowing you to modify data or prevent invalid operations. For instance, a BEFORE INSERT trigger could check if an email address is valid before saving it. An AFTER trigger runs post-event, often used for logging or cascading changes. Row-level triggers fire once for each affected row, while statement-level triggers run once per SQL statement, regardless of rows modified. For example, in PostgreSQL, a row-level trigger using FOR EACH ROW could update an audit log for every modified row, whereas a statement-level trigger might generate a summary report once after a bulk update.
Common use cases include auditing, enforcing complex business rules, or maintaining derived data. Suppose you have an orders table and want to log every price change. A trigger could automatically insert a record into an order_audit table with the old and new values, timestamp, and user. However, triggers should be used cautiously. Overuse can lead to hidden performance bottlenecks, especially with complex logic or cascading triggers. Debugging can also be challenging, as triggers execute implicitly. For example, a poorly designed trigger that calls other triggers might create unintended loops. Developers should prioritize clarity, test thoroughly, and document triggers to avoid maintenance issues.