Understanding Table Triggers and Auditing in SQL Server

Introduction to Table Triggers

In SQL Server, a table trigger is a stored procedure that runs automatically when certain events occur on a table. There are three types of triggers: INSERT, UPDATE, and DELETE. Each type of trigger has its own set of special tables, also known as “inserted” and “deleted,” that contain the data from the affected rows.

The Problem with the Current Trigger

In this article, we’ll explore why the current trigger is not working correctly when it comes to auditing changes made to the Email column in the AspNetUsers table.

-- Current trigger code
CREATE TRIGGER AspNetUsersEmail_trigger
ON AspNetUsers
AFTER INSERT, UPDATE
AS
IF ( UPDATE (Email) )
BEGIN
INSERT INTO [dbo].[AspNetUserEmailAudit]([UserId],[UserName],[Email],[NormalizedEmail],[FirstName],[LastName])
SELECT Id,[UserName],[Email],[NormalizedEmail],[FirstName],[LastName] FROM AspNetUsers
END;

The problem with this trigger is that it always inserts the entire AspNetUsers table, regardless of whether only one row has been updated. This means that even if no changes are made to the Email column, an audit entry will still be created.

The Solution: Using the Inserted Table

To fix this issue, we need to use the inserted table, which contains the data from the affected rows. In this case, we want to insert only the row that was updated.

-- New trigger code using inserted table
CREATE TRIGGER AspNetUsersEmail_trigger
ON AspNetUsers
AFTER INSERT, UPDATE
AS
IF ( UPDATE (Email) )
BEGIN
INSERT INTO [dbo].[AspNetUserEmailAudit]([UserId],[UserName],[Email],[NormalizedEmail],[FirstName],[LastName])
SELECT Id,[UserName],[Email],[NormalizedEmail],[FirstName],[LastName]
FROM inserted;
END;

By using the inserted table, we can ensure that only the row that was updated is inserted into the audit log.

Understanding the Inserted Table

The inserted table is a special table that contains the data from the affected rows when an INSERT or UPDATE trigger is fired. It’s similar to the deleted table, but it contains the new values instead of the old values.

-- Example of how inserted table works
CREATE TABLE #TempTable (
    Id INT PRIMARY KEY,
    Email VARCHAR(255)
);

INSERT INTO #TempTable (Id, Email) VALUES (1, 'new_email@example.com');

SELECT * FROM #TempTable;

-- Output:
-- Id  | Email
-- ----|--------
-- 1   | new_email@example.com

CREATE TABLE #TempTable (
    Id INT PRIMARY KEY,
    Name VARCHAR(255)
);

INSERT INTO #TempTable (Id, Name) VALUES (2, 'Old Value');

SELECT * FROM #TempTable;

-- Output:
-- Id  | Name
-- ----|--------
-- 2   | Old Value

CREATE TRIGGER TriggersExample
ON #TempTable
AFTER INSERT, UPDATE
AS
BEGIN
    INSERT INTO #TempTable2 (Id, Email)
    SELECT Id, Email FROM inserted;
END;

INSERT INTO #TempTable (Id, Email) VALUES (1, 'new_email@example.com');
SELECT * FROM #TempTable2;

-- Output:
-- Id  | Email
-- ----|--------
-- 1   | new_email@example.com

INSERT INTO #TempTable (Id, Name) VALUES (2, 'New Value');
SELECT * FROM #TempTable2;

-- Output:
-- Id  | Email
-- ----|--------
-- 2   | New Value

As you can see, the inserted table contains the new values from the affected rows.

Best Practices for Table Triggers

Here are some best practices to keep in mind when creating table triggers:

• Always use the inserted table instead of hardcoding the table name.
• Use the deleted table if you need to insert data into a table that was updated by another trigger.
• Avoid using table triggers as a substitute for regular database maintenance tasks. Instead, use them to automate specific business logic or auditing requirements.

Conclusion

In this article, we explored how to create a table trigger in SQL Server that audits changes made to the Email column in the AspNetUsers table. We discussed why the current trigger is not working correctly and showed how to fix it by using the inserted table. By following these best practices and understanding how table triggers work, you can create more effective and efficient database solutions for your organization.

Last modified on 2024-04-23