Triggers in SQL: Understanding Triggers and Trigger Examples in SQL Server

Last updated on February 10th, 2023 at 03:09 am

Triggers in SQL are special types of stored procedures that are automatically executed in response to specific events or changes within a database. In this article, we will explore triggers in SQL and provide a trigger example in SQL Server to help you understand how they work.

What are Triggers in SQL?

Triggers are database objects that are associated with a table or view and are automatically executed when specific actions occur. These actions can be any of the following:

• INSERT
• UPDATE
• DELETE
• TRUNCATE

SQL triggers are used to enforce business rules, keep data clean, and enforce referential constraints. They are used to implement security measures and perform audit trails on the data.

Advantages of Triggers in SQL

There are several benefits to using triggers in SQL, including:

• Automate Complex Processes: Triggers can be used to automate complex processes and reduce the need for manual intervention.
• Improved data integrity: SQL triggers can enforce business rules and maintain data integrity by preventing unauthorized data changes by the user.
• Improved performance: Triggers in SQL can help you to improve performance by reducing the need for manual processes and the amount of code required to maintain data integrity.
• Security: Triggers can also be used to enforce security measures by preventing unauthorized data access.

When to use Triggers in SQL

Triggers are most useful when a complex business process needs to be automated. For eg, if you want to keep track of all changes made to a specific table then you could use a trigger to automatically record the changes in a separate audit table.

Another example is when you need to enforce business rules. A trigger, for example, could be used to prevent users from deleting data in a specific table.

Syntax:

CREATE TRIGGER tr_audit_insert
ON dbo.YourTable
AFTER INSERT
AS
BEGIN
    INSERT INTO dbo.AuditTable (Column1, Column2, Column3)
    SELECT Column1, Column2, Column3
    FROM inserted;
END

Trigger Example in SQL Server

To demonstrate the use of triggers, let’s consider a scenario where we have a table called Orders and we want to keep track of all changes to the data in this table.

First, let’s create the Orders table:

CREATE TABLE Orders (
    OrderID INT PRIMARY KEY,
    CustomerID INT,
    OrderDate DATE,
    TotalAmount DECIMAL(10, 2)
);

Now, let’s insert some sample data into the Orders table:

INSERT INTO Orders (OrderID, CustomerID, OrderDate, TotalAmount)
VALUES (1, 1001, '2022-01-01', 1000),
       (2, 1002, '2022-02-01', 2000),
       (3, 1003, '2022-03-01', 3000),
       (4, 1004, '2022-04-01', 4000);

Let’s run the following SELECT query to see the data of the Orders table.

SELECT * from orders;

Now that we have the Orders table and sample data, let’s create a DML trigger that will keep track of all changes to the data in this table:

-- Create TRIGGER
CREATE TRIGGER tr_AuditOrders
ON Orders
AFTER INSERT, UPDATE, DELETE
AS
BEGIN
    DECLARE @EventType VARCHAR(20)
    SET @EventType = 
        CASE 
            WHEN EXISTS (SELECT * FROM inserted) AND EXISTS (SELECT * FROM deleted) THEN 'UPDATE'
            WHEN EXISTS (SELECT * FROM inserted) THEN 'INSERT'
            WHEN EXISTS (SELECT * FROM deleted) THEN 'DELETE'
        END

    IF @EventType = 'DELETE'
    BEGIN
        INSERT INTO AuditOrders (EventType, EventTime, OrderID, CustomerID, OrderDate, TotalAmount)
        SELECT @EventType, GETDATE(), OrderID, CustomerID, OrderDate, TotalAmount
        FROM deleted;
    END
    ELSE
    BEGIN
        INSERT INTO AuditOrders (EventType, EventTime, OrderID, CustomerID, OrderDate, TotalAmount)
        SELECT @EventType, GETDATE(), OrderID, CustomerID, OrderDate, TotalAmount
        FROM inserted;
    END
END;

Code Explanation:

In this example, the trigger tr_AuditOrders is created on the Orders table and will be executed after an INSERT, UPDATE, or DELETE statement is executed. The inserted and deleted virtual tables are used to retrieve the data that was affected by the DML operation. The type of event that triggered the trigger (INSERT, UPDATE, or DELETE) is determined based on the existence of data in the inserted and deleted tables. The event type, time, and affected data are then inserted into the AuditOrders table.

Let’s create the AuditOrders table to store the audit data:

CREATE TABLE AuditOrders (
    EventType VARCHAR(20),
    EventTime DATETIME,
    OrderID INT,
    CustomerID INT,
    OrderDate DATE,
    TotalAmount DECIMAL(10, 2)
);

Now, let’s test the trigger by inserting, updating, and deleting data in the Orders table:

-- Insert new record
INSERT INTO Orders (OrderID, CustomerID, OrderDate, TotalAmount)
VALUES (5, 1005, '2022-05-01', 5000);

-- Update record
UPDATE Orders
SET TotalAmount = TotalAmount + 1000
WHERE OrderID = 5;

-- Delete record
DELETE FROM Orders
WHERE OrderID = 5;

Let’s query the AuditOrders table to see the audit data that was generated by the trigger:

SELECT * FROM AuditOrders;

This query will show the event type, time, and data that was affected by the DML operations on the Orders table.

Here is an example of what the results of this query might look like:

AuditOrders Table:

Types of triggers in SQL

In SQL, there are two main types of triggers: DDL (Data Definition Language) triggers and DML (Data Manipulation Language) triggers.

01. DDL Triggers

DDL triggers in SQL are used to track database schema changes such as the creation or modification of tables, views, or stored procedures. DDL triggers are commonly used for auditing and security purposes because they allow you to track who and what changes are being made to the database.

DDL triggers are fired in response to CREATE, ALTER, and DROP statements in SQL.

Here is an example of a DDL trigger in the SQL Server:

CREATE TRIGGER tr_audit_ddl
ON DATABASE
FOR CREATE_TABLE
AS
BEGIN
    INSERT INTO dbo.AuditTable (EventType, EventTime, ObjectName)
    VALUES ('CREATE_TABLE', GETDATE(), OBJECT_NAME(@@PROCID));
END

In this example, The trigger is created on the database and will be executed whenever a CREATE TABLE statement is executed.
The @@PROCID function returns the ID of the stored procedure that caused the event. The event type and time, as well as the name of the newly created object, are then inserted into the dbo.AuditTable table.

02. DML Triggers

DML triggers are used to track changes to data within a table, such as INSERT, UPDATE, or DELETE operations. DML triggers are typically used for maintaining data integrity and enforcing business rules.

In the above examples, we have already created a DML trigger tr_AuditOrders in SQL Server:

How To Create SQL Server Trigger For Update Event?

Let’s create a SQL server trigger for an update event that will get executed once a row is updated in a table.

Before creating a trigger, let’s first create sample tables for examples. Let’s create a table “tblEmployee” to keep employee personnel information and a table “tblEmployeeHistory” to contain each modified record.

SQL Server Script To Create “tblEmployee” Table

-- Creating a new table "tblEmployee"
CREATE TABLE tblEmployee
(
  [Id] INT PRIMARY KEY,
  [Name] NVARCHAR(30),
  [Gender] VARCHAR(10),
  [City] VARCHAR(50),
  [DeptId] INT
)

-- Inserting data into the "tblEmployee" table.

INSERT INTO tblEmployee VALUES 
(101,'Shekh Ali','Male','Delhi', 1),
(102,'Mark Adam','Male','New York', 1),
(103,'Julie Folden','Female','Chicago', 2);

-- SQL Script to create a table "tblEmployeeHistory"

CREATE TABLE tblEmployeeHistory
(
  [Id] INT IDENTITY(1,1) PRIMARY KEY,
  [AuditData] NVARCHAR(500),
  [Date] DATETIME
)

-- Select records from the following tables

SELECT *  FROM tblEmployee;

SELECT *  FROM tblEmployeeHistory;

After executing the SQL statements referred to above, the following tables will be created in the database.

In general, DML triggers create two special (virtual) magic tables called INSERTED and DELETED with the same structure or scheme as their base table.

The INSERTED table contains newly inserted records; however, the DELETED table contains old records after updating or deleting rows in a specific table.

In the SQL server trigger for the update, the temporarily inserted and deleted tables are created.

Syntax: SQL Trigger For Update Query

-- SQL script for creating a DML trigger for update events.
-- Author: Shekh Ali

CREATE TRIGGER TR_TblEmployee_ForUpdate ON tblEmployee
FOR UPDATE 
AS
BEGIN
 -- Declare variables to hold the old and updated record

      DECLARE @Id INT, @OldDeptId INT;
      DECLARE @OldName NVARCHAR(30), @OldGender NVARCHAR(10), @OldCity NVARCHAR(30);
     
      -- Variable to build the audit string

      DECLARE @AuditString NVARCHAR(1000);

	  -- Select the corresponding Id from the inserted table

	  Select Top 1 @Id = Id FROM INSERTED;

	   -- Select the corresponding row from the deleted table

            SELECT @OldName = Name, @OldGender = Gender, 
            @OldCity = City, @OldDeptId = DeptId
            FROM DELETED WHERE Id = @Id;

       -- Build the audit string dynamically           
            Set @AuditString = 'Employee with Id = ' + Cast(@Id as NVARCHAR(4)) + ' changed'+
			+' Name: ' +@OldName +' Gender: ' +' City: '+@OldCity +' DeptId: '+ Cast(@OldDeptId as NVARCHAR(4));

	  INSERT INTO tblEmployeeHistory VALUES(@AuditString, getdate());
END

Steps to create SQL update trigger

1. 1. First, To create a trigger, use the  CREATE TRIGGER  keywords, then specify the unique trigger name (Example: TR_TblEmployee_ForUpdate).
   2. Second, the options FOR, INSTEAD OF, or AFTER specify when the trigger will be executed either before or after single or multiple rows are modified.
   3. Then use the keyword  ON  to specify the name of the table associated with the trigger.
   4. Finally, specify the trigger body started from  AS BEGIN  to  END  which contains one or more SQL statements to be executed once trigger fired.

After execution of the above-mentioned SQL statements, a new SQL trigger for an update command will be created in the table-level trigger directory.

Please see the below image for reference.

Update record in a database table

Let’s try to update a record in the “tblEmployee” table.

-- Update query
UPDATE tblEmployee SET City='Mumbai' WHERE Id=101;

Immediately after executing the UPDATE statement, the AFTER UPDATE trigger launches, and the new and old data will be stored in the INSERTED and DELETED magic tables.

Now we will be able to see the updated record in the”tblEmployee” table and the old record in the “tblEmployeeHistory” table inserted by the after update trigger.

How To Create SQL Server Instead Of Update Trigger?

In SQL Server, the INSTEAD OF UPDATE Trigger is executed automatically instead of the actual UPDATE event on a table or a view.

Suppose we have an INSTEAD OF UPDATE trigger on a table “tblEmployee”.
When we attempt to update a row of the “tblEmployee” table, the following trigger will be executed automatically instead of the actual Update event.

Let’s put one condition inside the trigger, if someone tries to update the Id column from the “tblEmployee” table, then the such record must not be allowed to update but raised an error message.

The update query will be only allowed to execute If any other columns are updated rather than the ID column.

Syntax: SQL Instead of update trigger

Following are the SQL statements used to create Instead of update triggers.

-- SQL script to create Instead Of Trigger.
-- Author: Shekh Ali

CREATE TRIGGER [dbo].[Tr_tblEmployee_InsteadOfUPDATE]
       ON [dbo].[tblEmployee]
INSTEAD OF UPDATE
AS
BEGIN
       SET NOCOUNT ON;
 
       DECLARE @EmployeeId INT,@DeptId INT 
	   DECLARE @Name VARCHAR(50), @City VARCHAR(50), @Gender VARCHAR(50)
 
       SELECT @EmployeeId = Id,
              @Name   = [Name],
              @City   = City,
			  @Gender = Gender,
			  @DeptId = DeptId
       FROM INSERTED ;-- Magic Table
 
       IF UPDATE(Id)
       BEGIN
              RAISERROR('EmployeeId cannot be updated.', 16 ,1)
              ROLLBACK
              INSERT INTO tblEmployeeHistory
              VALUES( 'EmployeeId ' + Cast(@EmployeeId as NVARCHAR(5)) + ' cannot be updated.',GETDATE())
       END
       ELSE
       BEGIN
              UPDATE [tblEmployee]
              SET [Name] = @Name,
              City = @City,
			  Gender = @Gender,
			  DeptId = @DeptId
              WHERE Id = @EmployeeId
       END
END

Let’s try to update the Id column in the table “tblEmployee”.

-- Update query
UPDATE tblEmployee SET Id=105 WHERE Id=101;

The following error message will be shown when the Id field is updated in the tblEmployee table.

The “tblEmployeeHistory” table is also updated through the SQL Instead of Update trigger

Example: SQL For Insert Trigger

Create an INSERT trigger that will add the newly added employee Id to the History table.

-- SQL FOR INSERT TRIGGER EXAMPLE
CREATE TRIGGER [dbo].[Tr_tblEmployee_INSERT]
       ON [dbo].[tblEmployee]
FOR INSERT
AS
BEGIN
       SET NOCOUNT ON;
 
       DECLARE @EmployeeId INT
 
       SELECT @EmployeeId = INSERTED.Id       
       FROM INSERTED

       INSERT INTO tblEmployeeHistory
	   VALUES( 'EmployeeId ' + Cast(@EmployeeId as NVARCHAR(5))
	   + ' Inserted.',GETDATE())
END

Let’s try to insert the following employee record into the “tblEmployee” table.

INSERT INTO tblEmployee VALUES (104,'Rajat Sharma','Male','Delhi', 1);

Once we execute the above Insert statement, the trigger will insert the employee id detail in the tblEmployeeHistory table.

How to Create Delete Trigger In SQL Server?

Let’s Create a trigger that will not allow the Delete operation on the table “tblEmployee”.

CREATE TRIGGER Tr_tblEmployee_Delete
ON tblEmployee
FOR DELETE
AS
BEGIN
  PRINT 'DELETE OPERATION IS RESTRICTED.'
  ROLLBACK TRANSACTION
END

Let’s try to DELETE the record of the employee with Id is 101 in the table “tblEmployee”.

-- Delete employee with Id 101
DELETE FROM tblEmployee WHERE ID = 101;

When we try to execute the above Delete statement, it gives us the following error.

Limitations of triggers In SQL Server

• CREATE TRIGGER must be the first statement in the batch and can apply to only one table.
• A trigger is created only in the current database; however, a trigger can reference objects outside the current database.
• If the trigger schema name is specified to qualify the trigger, qualify the table name in the same way.
• The same trigger action can be defined for more than one user action (for example, INSERT and UPDATE) in the same CREATE TRIGGER statement.
  INSTEAD OF DELETE/UPDATE triggers can’t be defined on a table that has a foreign key with a cascade on DELETE/UPDATE action defined.

Conclusion:

In this article, we provided a comprehensive overview of triggers in SQL Server, including a trigger example in SQL. We also demonstrated how to implement triggers in SQL Server, and outlined the advantages of using triggers in your database.

FAQ

References: MSDN- CREATE TRIGGER (MSDN)

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Shekh Ali

Meet Sheikh Ali, a skilled Software engineer and passionate a blogger. He enjoys sharing his expertise in C#, Java, SQL, Design Patterns, and other related topics through his writing. When he's not coding, Sheikh Ali can often indulge in his passions for reading books and fitness training.

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