SQL-Server Triggers  «Prev  Next»

Lesson 2 What is a trigger?
Objective SQL Server triggers and how can it be used

SQL Server Trigger Utility

As a SQL Server Administrator, utilizing SQL Server triggers effectively can streamline various database operations and enforce business rules. Here are the top three use cases for SQL Server triggers:
  1. Data Validation and Integrity: One of the primary use cases for SQL Server triggers is to enforce data validation and integrity rules that go beyond the constraints available through standard SQL. For instance, a trigger can be used to check the validity of data before it's inserted or updated in a table. This might involve checking if data meets certain criteria, falls within a specified range, or adheres to complex business rules that cannot be enforced through simple constraints. By implementing these checks in a trigger, you ensure that only valid data is stored in your database, maintaining its integrity.
  2. Audit Logging: Triggers are highly useful for audit logging. You can use them to automatically record changes made to data in a table. For example, a trigger can be set up on a table to log all INSERT, UPDATE, and DELETE operations, capturing key information such as what was changed, who made the change, and when the change occurred. This audit trail is invaluable for tracking user activities, understanding the history of data modifications, and complying with regulatory requirements that mandate detailed records of data transactions.
  3. Automating System Responses: Triggers can automate responses to certain events within your database. For instance, a trigger could be configured to send an alert, email, or initiate a workflow when specific conditions are met, such as a record being updated or deleted. This automated response mechanism is particularly useful in scenarios where immediate action is required, such as notifying administrators of critical changes, triggering data synchronization processes, or enforcing business logic that extends beyond the database, like updating related systems or triggering external processes.

Each of these use cases demonstrates the versatility and power of SQL Server triggers in maintaining data integrity, ensuring compliance through auditability, and automating responses to database events. As an administrator, leveraging triggers effectively can significantly enhance the robustness and efficiency of your database management.

What is a Trigger in SQL-Server?

A trigger is a SQL Server utility that causes an event to fire when data in a table is inserted, updated, or deleted. Triggers are fired according to specifications made by the SQL Server programmer. For example, you might specify that a trigger will fire when the Salary column in the Employees table is updated. Triggers are an important tool for enforcing business rules, because they can be set to fire automatically when potential changes to your table conflict with established business rules. Triggers are set within the database rather than within a particular application, which means:
  1. Triggers will fire regardless of what application generates the request, which increases your ability to enforce business rules.
  2. When a business rule changes, you need to change the trigger only once.

Triggers and errors

A trigger will not fire when the requesting Transact-SQL statement produces an error, such as a primary key or syntax violation. You can use this restriction to control when triggers are fired. For example, you could specify that an error will be raised when the Salary column in the Employees table is greater than $100,000. In this case, the trigger will be fired, but no action will be taken.
  1. Nesting triggers: When triggers are nested, one sets off another, and so on. Triggers can be nesting up to 16 levels deep. The nesting function can be turned off, or the number of levels can be decreased, by using the sp_configure system procedure.
  2. Cascading triggers: A cascade is a series of updates or deletes invoked by a trigger. For example, you can create a trigger that fires when an employee is deleted from the database. However, when the employee is deleted, you probably do not want his or her information stored in all the other tables that contain the employee's ID. In this situation, the trigger can perform a cascade of deletes so that there is no trace of the employee in any table.
    In the next lesson, the different types of triggers will be discussed.

SQL Server has two kinds of transaction triggers: 1) instead of triggers and 2) after triggers. They differ in their purpose, timing, and effect, as detailed in the table below.

Trigger Type Comparison
Trigger Type Comparison

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