How Can I Modify Identity Columns in SQL Server?

Updating Identity Columns in SQL Server: A Comprehensive Guide

When working with SQL Server databases, you may encounter a situation where you need to modify the identity column. In this article, we will explore the options available to address this requirement and provide a thorough explanation of the limitations and alternatives.

The Inability to Update Identity Columns

Unlike other columns in a SQL Server table, identity columns cannot be updated directly using an update statement. This restriction is due to the nature of identity columns, which are designed to automatically increment with each new record inserted into the table.

Alternatives to Updating Identity Columns

Although direct updates are not possible, there are alternative methods to achieve similar results depending on your specific requirements:

1. Updating Identity Values for New Records

If you need to ensure that future records inserted into a table start with a specific identity value, you can use the DBCC CHECKIDENT command. This command allows you to reset the current identity value and specify a new starting value.

Syntax:

DBCC CHECKIDENT('tableName', RESEED, NEW_RESEED_VALUE)

2. Updating Identity Values for Existing Records

To update the identity value of existing records, you can utilize the IDENTITY_INSERT property. This property allows you to explicitly specify the identity value when inserting new records.

Syntax:

SET IDENTITY_INSERT YourTable {ON|OFF}

Example:

-- Turn on IDENTITY_INSERT to allow explicit identity value insertion
SET IDENTITY_INSERT YourTable ON
GO
-- Insert a new record with the desired identity value
INSERT INTO YourTable(IdentityCol, otherCol) VALUES(13,'myValue')
GO
-- Delete the old record with the existing identity value
DELETE FROM YourTable WHERE ID=3
GO
-- Turn off IDENTITY_INSERT to resume automatic identity value increment
SET IDENTITY_INSERT YourTable OFF

By leveraging these techniques, you can effectively address the challenges associated with updating identity columns in SQL Server.

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