SQL AFTER UPDATE Trigger

Introduction to SQL AFTER UPDATE Trigger

AFTER UPDATE Trigger in SQL is a stored procedure on a database table that gets invoked or triggered automatically after an UPDATE operation gets successfully executed on the specified table. For uninitiated, the UPDATE statement is used to modify data in existing rows of a data table. The AFTER UPDATE trigger is helpful in scenarios where we want to record the time of status change or record changes in the rating field based on the modification in values of some other field etc. It basically helps us in logging, auditing, and tracking changes.

Syntax

The basic syntax for writing SQL AFTER UPDATE Trigger in SQL is as follows :

CREATE TRIGGER [schema_name. ] trigger_name ON table_name
AFTER UPDATE
AS
BEGIN
[SET NOCOUNT {ON/OFF}] {SQL statements}
END

Parameters

The parameters used in the above-mentioned syntax are as follows :

schema_name: This is the name of the schema in which we will be creating the new trigger. You may ignore it as a schema name is optional. By default, triggers will be created in the schema you are currently working in.

trigger_name: This is the name of the trigger which you want to create.

table_name: This is the name of the table to which the trigger will be applied.

SQL statements: SQL statements form the body of the trigger. These are a set of SQL operations that will be performed once the AFTER UPDATE trigger is invoked.

Examples to Implement SQL AFTER UPDATE Trigger

let us try a few examples to understand it in detail:

In order to illustrate the functionality of the AFTER UPDATE trigger, let us create a dummy table called “books_audit_table”. This table contains details pertaining to books available in a library. We can use the following code snippet to create the said table.

Code:

CREATE TABLE books_audit_table (
book_id INT NOT NULL IDENTITY PRIMARY KEY,
title VARCHAR(100)  NOT NULL,
author_name  VARCHAR(100),
genre VARCHAR(100),
updated_at DATETIME,
status VARCHAR(100)
);

Output:

The query returned successfully. Next, using the following INSERT query insert a few records in the table to work with.

Code:

INSERT INTO [practice_db].[dbo].[books_audit_table] ([title] ,[author_name] ,[genre] ,[updated_at] ,[status])
VALUES
('The Choice','Edith Eva Eger','Memoir',NULL, NULL),
('Deep Work','Carl Newport','Self Help',NULL, NULL),
('A Man Called Ove','Fredrik Backman','Fiction',NULL, NULL),
('When Breath Becomes Air','Paul Kalanithi','Memoir',NULL, NULL),
('Man Search for Meaning','Viktor Frankl','Memoir',NULL, NULL),
('The Third Pillar','Raghuram Rajan','Economics',NULL, NULL)
GO

Output:

we have successfully created the “books_audit_table”. Now we are all set to try a few examples based on the AFTER UPDATE trigger.

Example #1

Step 1: Create a trigger in SQL, which automatically updates the date and time of borrowing a book from the collection.

Code:

CREATE TRIGGER update_trigger ON books_audit_table
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
UPDATE books_audit_table set updated_at = GETDATE()
from books_audit_table b
INNER JOIN inserted i on b.book_id=i.book_id
AND i.status = 'Borrowed'
END
GO

Output:

Step 2: The query has successfully created the update_trigger. We can check the newly created trigger in the object explorer.

Step 3: Now, let us update the status of the first book to ‘Borrowed’ using the following statement.

Code:

UPDATE books_audit_table
SET Status='Borrowed'
WHERE book_id = 1;

Output:

Step 4: Since the update query was successful, update_trigger must have been automatically invoked and the updated_at column must have been automatically populated with the current timestamp. The following select query can be used to check the same.

Code:

SELECT * FROM books_audit_table;

Output:

Step 5: We can observe in the image that for book_id = 1, the updated_at column has been populated. Let us try one more example based on the same concept. This time let us do it for book_id = 5.

Code:

UPDATE books_audit_table
SET Status='Borrowed'
WHERE book_id = 5;

Output:

Step 6:

Explanation: Here as well, the update_trigger got automatically invoked and the updated_at column is updated to the current timestamp.

Example #2

Suppose two new columns such as borrowed_at and returned_at have been added to the books_audit_table. These columns have to be populated with a timestamp when the status of the book changes to “Borrowed” or “Returned”.

Step 1: You may use the following ALTER statements to modify the table.

Code:

ALTER TABLE books_audit_table
DROP COLUMN updated_at;
ALTER TABLE books_audit_table
ADD borrowed_at DATETIME;
ALTER TABLE books_audit_table
ADD returned_at DATETIME;

Output:

Step 2: The modified table looks something as follows:

Step 3: Next, let us create an after update trigger called “update_trigger_new” using the following query.

Code:

CREATE TRIGGER update_trigger_new ON books_audit_table
AFTER UPDATE
AS
BEGIN
SET NOCOUNT ON;
UPDATE books_audit_table set borrowed_at = GETDATE()
FROM books_audit_table b
INNER JOIN inserted i on b.book_id=i.book_id
AND i.status = 'Borrowed'
UPDATE books_audit_table set returned_at = GETDATE()
FROM books_audit_table b
INNER JOIN inserted i on b.book_id=i.book_id
AND i.status = 'Returned'
END
GO

Output:

The trigger has been successfully created. It can be seen in the object explorer.

Step 4: Next, let us update the status of book_id 3 to “Returned ”.

Code:

UPDATE books_audit_table
SET Status='Returned'
WHERE book_id = 3;

Output:

Step 5: The highlighted row shows the changes made by the update query and the trigger. It’s fun right. Let us try the following update query.

Code:

UPDATE books_audit_table
SET status = 'Returned'
WHERE book_id = 1;

Output:

Explanation: Wow, all the necessary changes have been made by the trigger.

Conclusion

AFTER UPDATE Trigger is a kind of trigger in SQL that will be automatically fired once the specified update statement is executed. It can be used for creating audit and log files which keep details of last update operations on a particular table.

Recommended Article

This is a guide to SQL AFTER UPDATE Trigger. Here we discuss an introduction to SQL AFTER UPDATE Trigger, examples for better understanding. You can also go through our other related articles to learn more –

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