Understanding the OUTPUT Clause in SQL Server

SQL Server provides a powerful feature called the OUTPUT clause, which allows developers to return information from the rows affected by an INSERT, UPDATE, DELETE, or MERGE statement. This clause can be used to return the values of the columns affected by the statement, which can be inserted into a table, used in a SELECT statement, or both. In this article, we will explore the OUTPUT clause and how it can be used to backup or save data while updating.

What is the OUTPUT Clause?

The OUTPUT clause is a SQL Server feature that allows you to return information from the rows affected by an INSERT, UPDATE, DELETE, or MERGE statement. The returned information can be inserted into a table, used in a SELECT statement, or both. The OUTPUT clause can be useful for auditing changes to the data, keeping a history of updates, or for creating a backup of the data before making changes.

How does the OUTPUT Clause work with UPDATE statements?

In the case of an UPDATE statement, the OUTPUT clause can return the values of the columns before and after the update. The keywords “deleted” and “inserted” are used to refer to the old and new values of the columns, respectively. For example, the following query updates the column1 in table1 with the column2 from table2 by joining the two tables on column3 from table1 and column4 from table2, and also returns the old and new values of column1, column3, and column4 into a temporary table named @TempTable.

BEGIN TRANSACTION

DECLARE @TempTable TABLE (old_column1 INT, new_column1 INT, column3 INT, column4 INT)

UPDATE t1
SET t1.column1 = t2.column2
OUTPUT deleted.column1, inserted.column1, inserted.column3, inserted.column4 
INTO @TempTable
FROM table1 as t1
INNER JOIN table2 as t2
ON t1.column3 = t2.column4

COMMIT TRANSACTION

How OUTPUT clause can be used to backup or save data while updating

The OUTPUT clause can be used to backup or save data while updating. By using the OUTPUT clause with an UPDATE statement, you can return the old and new values of the columns affected by the update and insert them into a table. This allows you to keep a history of the changes made to the data, which can be useful for auditing or troubleshooting purposes.

For example, by using the above query you can save the old values of the rows that are being updated in a temporary table before the actual update happens. Once you confirm that the update statement is working as expected, you can then commit the transaction and make the changes permanent in the database. And if there is any issue and you want to rollback the changes, you can use ROLLBACK TRANSACTION statement instead of COMMIT TRANSACTION.

It’s worth mentioning that output clause can only be used in the context of a single statement like above example and not with subqueries or other statements like triggers.

In conclusion, the OUTPUT clause in SQL Server is a powerful feature that allows developers to return information from the rows affected by an INSERT, UPDATE, DELETE, or MERGE statement. It can be used to backup or save data while updating and is a useful tool for auditing changes to the data, keeping a history of updates, or for creating a backup of the data before making changes. By using the OUTPUT clause with an UPDATE statement, developers can ensure that data is not lost or corrupted during updates and that a history of changes is kept. This can be particularly useful in scenarios where data integrity and recovery are critical.

It’s important to note that while the OUTPUT clause can be very useful in certain situations, it should be used with caution, especially when working with large datasets. As with any feature, it’s important to thoroughly test your code and be mindful of performance implications before deploying it in a production environment.

In summary, the OUTPUT clause is a powerful feature in SQL Server that allows developers to return information from the rows affected by an INSERT, UPDATE, DELETE, or MERGE statement, it can be used to backup or save data while updating, auditing changes to the data, keeping a history of updates, or for creating a backup of the data before making changes. It should be used with caution and thoroughly tested before deploying in production environment.