Managing Identity Columns in SQL Server

Identity columns in SQL Server are used to automatically generate a unique identifier for each row in a table. These columns are often used as primary keys, providing a simple and efficient way to uniquely identify records. However, there may be situations where you need to update or reset the identity column. In this tutorial, we’ll explore how to manage identity columns effectively.

Understanding Identity Columns

Before diving into updating or resetting identity columns, it’s essential to understand how they work. When a table with an identity column is created, SQL Server automatically assigns a unique identifier to each new row inserted into the table. The value of the identity column starts from a specified seed value and increments by a specified increment value for each new row.

Updating Existing Records

Updating existing records in an identity column can be challenging because SQL Server does not allow direct updates to identity columns using the UPDATE statement. However, you can use the SET IDENTITY_INSERT statement to temporarily enable explicit values to be inserted into the identity column of a table.

Here’s an example:

-- Set Identity insert on so that value can be inserted into this column
SET IDENTITY_INSERT YourTable ON;
GO

-- Insert the record which you want to update with new value in the identity column
INSERT INTO YourTable(IdentityCol, otherCol) VALUES(13,'myValue');
GO

-- Delete the old row of which you have inserted a copy (above) (make sure about FK's)
DELETE FROM YourTable WHERE ID=3;
GO

-- Now set the idenetity_insert OFF to back to the previous track
SET IDENTITY_INSERT YourTable OFF;

Resetting Identity Columns

If you need to reset the identity column, for example, to start from a new seed value, you can use the DBCC CHECKIDENT statement. This command checks the current identity value for the table and changes it if necessary.

Here’s an example:

-- Reseed the identity column to start from 1
DBCC CHECKIDENT ('YourTable', RESEED, 1);

Replacing Existing Data with Updated Identity Values

In cases where you need to replace existing data with updated identity values, a more complex approach is required. This involves temporarily storing the data in a temporary table, deleting the original data, and then reinserting it with the updated identity values.

Here’s an example:

-- Set table to allow identity to be inserted
SET IDENTITY_INSERT yourTable ON;
GO

-- Insert everything into a temp table
SELECT * 
INTO #tmpYourTable
FROM yourTable;

-- Clear your table
DELETE FROM yourTable;

-- Insert back all the values with the updated ID column
INSERT INTO yourTable (IDCol, OtherCols)
SELECT ID+1 as updatedID -- Put any other update logic to the ID here
, OtherCols 
FROM #tmpYourTable;

-- Drop the temp table
DROP TABLE #tmpYourTable;

-- Put identity back to normal
SET IDENTITY_INSERT yourTable OFF;
GO

Best Practices

• Always back up your database before making significant changes to your tables.
• Be cautious when updating or resetting identity columns, especially if there are foreign key constraints involved. Ensure that you understand the impact on related data and make necessary adjustments to avoid data inconsistencies.
• Use DBCC CHECKIDENT and SET IDENTITY_INSERT judiciously, as these commands can have significant implications for your database’s integrity and performance.

By following these guidelines and examples, you’ll be able to manage identity columns in SQL Server effectively, ensuring the integrity and consistency of your database.