Understanding SQL Triggers: A Practical Guide with a New Example

Introduction

Tiggers are commonly used in SQL Server to automate data updates. This introduction article explains the basics of triggers in SQL, the types of triggers, and how to implement triggers in SQL Server.

What is Trigger in SQL Server?

A SQL trigger is a database object which fires when an event occurs in a database. For example, we can execute a SQL query that will "do something" in a database when a change occurs on a database table, such as when a record is inserted, updated, or deleted. For example, a trigger can be set on a record insert in a database table. For example, if you want to increase the blogs count in the Reports table when a new record is inserted in the Blogs table, we can create a trigger on the Blogs table on INSERT and update the Reports table by increasing the blog count to 1.

Types of Triggers in SQL Server

There are two types of triggers:

DDL Trigger
DML Trigger

DDL Triggers in SQL Server

The DDL triggers are fired in response to DDL (Data Definition Language) command events that start with Create, Alter, and Drop, such as Create_table, Create_view, drop_table, Drop_view, and Alter_table.

Code of a DDL Trigger

CREATE TRIGGER safety
ON DATABASE
FOR CREATE_TABLE, ALTER_TABLE, DROP_TABLE
AS
    PRINT 'You cannot create, drop, or alter tables in this database'
    ROLLBACK;

When we create, alter, or drop any table in a database, then the following message appears,

DDL Triggers in SQL Server

DML Triggers in SQL Server

The DML triggers are fired in response to DML (Data Manipulation Language) command events that start with Insert, Update, and Delete. Like insert_table, Update_view and Delete_table.

CREATE TRIGGER deep
ON emp
FOR INSERT, UPDATE, DELETE
AS
    PRINT 'You cannot insert, update, or delete from this table i'
    ROLLBACK;

When we insert, update, or delete a table in a database, then the following message appears,

DML Triggers in SQL Server

Logon Triggers in SQL Server

In SQL Server, logon triggers are special types of triggers that are designed to execute automatically in response to a user's logon event. When a user connects to a SQL Server instance, the logon trigger fires before the user's session is established, allowing you to perform certain actions or enforce specific rules based on the user's login.

CREATE TABLE dbo.LogonAudit (
    LogonAuditID INT IDENTITY(1,1) PRIMARY KEY,
    LoginName NVARCHAR(50),
    LoginTime DATETIME,
    ClientHost NVARCHAR(50)
);
GO

CREATE TRIGGER LogonAuditTrigger
ON ALL SERVER WITH EXECUTE AS 'sa'
FOR LOGON
AS
BEGIN
    -- Insert logon event details into an audit table
    INSERT INTO dbo.LogonAudit (LoginName, LoginTime, ClientHost)
    VALUES (ORIGINAL_LOGIN(), GETDATE(), HOST_NAME())
END;
GO

In this example, we create a logon trigger called LogonAuditTrigger that fires for all logon events on the server. The trigger is configured to execute under the context of the sa account, which requires appropriate permissions.

Within the trigger's body, we perform an action of inserting logon event details into an audit table called LogonAudit. The ORIGINAL_LOGIN() function retrieves the login name of the user who is attempting to log in. GETDATE() returns the current date and time when the logon occurs. HOST_NAME() retrieves the client host name or IP address from which the logon originates.

By using this logon trigger, each time a user logs in to the SQL Server instance, the trigger will capture and store the login name, login time, and client host information in the LogonAudit table, providing an audit trail of logon events.

Types of DML triggers

There are two types of DML Triggers in SQL Server.

AFTER Triggers

AFTER triggers are executed after the action of an INSERT, UPDATE, or DELETE statement.

create trigger insertt  
on emp  
after insert  
as  
begin  
insert into empstatus values('active')  
end

AFTER Triggers in SQL

INSTEAD Of Triggers

It will tell the database engine to execute the trigger instead of the statement. For example, an insert trigger executes when an event occurs instead of the statement that would insert the values in the table.

CREATE TRIGGER instoftr  
ON v11  
INSTEAD OF INSERT  
AS  
BEGIN  
INSERT INTO emp  
SELECT I.id, I.names  
FROM INSERTED I  
   
INSERT INTO emp1values  
SELECT I.id1, I.name1  
FROM INSERTED I  
END

When we insert data into a view by the following query, it inserts values in both tables.

insert into v11 values(1,'d','dd')

You can see both tables by the following query.

select * from emp  
select * from emp1values

INSTEAD Of Triggers in SQL

Summary

In this article, you learn about triggers in SQL and how to implement triggers in SQL Server.