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This document provides complete and detailed notes on SQL Triggers, including types, creation, execution flow, and real-world applications. What you will learn: Introduction to SQL Triggers Types of triggers (BEFORE, AFTER, INSTEAD OF) Creating and executing triggers Advantages and disadvantages Real-world use cases
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1. Introduction to SQL Triggers SQL Triggers are special types of stored programs that automatically execute in response to specific events on a table or view. These events include operations such as INSERT, UPDATE, and DELETE. Unlike stored procedures or functions, triggers are not called manually. Instead, they are activated automatically when a defined event occurs. In modern database systems, maintaining data consistency and enforcing business rules is critical. Triggers help achieve this by ensuring that certain actions are automatically performed whenever data changes. For example, a trigger can automatically update a log table whenever a new record is inserted into the main table. Triggers operate at the database level, which means they are independent of the application layer. This makes them reliable for enforcing rules because they execute regardless of how the data is modified. Understanding SQL triggers is important for database automation, auditing, and maintaining data integrity. They are widely used in enterprise systems where automatic responses to data changes are required.
This ensures that salary values below a minimum threshold are adjusted automatically.
5. AFTER Trigger An AFTER trigger is executed after the triggering event has been completed. It is commonly used for logging and auditing purposes. Example: CREATE TRIGGER log_update AFTER UPDATE ON employees FOR EACH ROW BEGIN INSERT INTO employee_log VALUES (OLD.id, OLD.salary, NEW.salary); END; This trigger records changes whenever an employee's salary is updated. 6. INSTEAD OF Trigger An INSTEAD OF trigger is used to replace the default action of a query. It is often used with views where direct modifications are not allowed. For example, an INSTEAD OF INSERT trigger can define custom logic for inserting data into multiple tables through a view.
7. Creating a Trigger Triggers are created using the CREATE TRIGGER statement. Syntax: CREATE TRIGGER trigger_name AFTER INSERT ON table_name FOR EACH ROW BEGIN -- SQL statements END; Creating triggers allows automation of database operations. 8. Trigger Execution Flow When an event occurs, the database checks if any triggers are associated with that event. If a trigger exists, it is executed automatically. The flow includes event detection, trigger execution, and completion of the operation. This ensures that all defined rules are applied consistently. 9. Advantages of Triggers Automatic execution Enforces business rules Maintains data integrity Useful for auditing and logging Reduces manual intervention 10. Disadvantages of Triggers Hard to debug Can impact performance Hidden logic may confuse developers Complex triggers can be difficult to maintain
