Modifying Existing Columns to Include Identity Properties

In database design, it’s common to encounter situations where an existing column needs to be modified to include identity properties. This could be due to changes in requirements or the need for auto-incrementing values in a specific column. However, SQL Server does not directly support altering an existing column to add an identity property through a simple ALTER TABLE statement.

To achieve this, several approaches can be taken, each with its own set of considerations and potential drawbacks. The most straightforward methods involve either creating a new table with the desired identity column and then transferring data from the old table to the new one or adding a new column with an identity property and dropping the original column after updating it.

Creating a New Table with Identity

One approach is to create a new table that includes the identity column, transfer all existing data into this new table, drop the original table, and then rename the new table to match the name of the original table. This method ensures that you can retain the existing data values but requires careful handling to avoid losing any data.

Here’s an example of how to implement this:

CREATE TABLE Tmp_Names (
    Id INT NOT NULL IDENTITY(1, 1),
    Name VARCHAR(50) NULL
);

SET IDENTITY_INSERT Tmp_Names ON;
INSERT INTO Tmp_Names (Id, Name)
SELECT Id, Name FROM Names;
SET IDENTITY_INSERT Tmp_Names OFF;

DROP TABLE Names;
EXEC sp_rename 'Tmp_Names', 'Names';

Adding a New Column with Identity

Another approach is to add a new column with an identity property and then drop the original column. However, this method means you cannot retain the existing data values on the newly created identity column, as it will start sequencing from 1.

ALTER TABLE Names
ADD Id_new INT IDENTITY(1, 1);

ALTER TABLE Names DROP COLUMN ID;

EXEC sp_rename 'Names.Id_new', 'ID', 'COLUMN';

Using SQL Server’s ALTER TABLE…SWITCH Statement

For large tables where modifying data pages could be time-consuming or inefficient, SQL Server provides the ALTER TABLE...SWITCH statement. This allows for changing the schema of a table without altering its data, which can be particularly useful when adding an identity column to an existing table.

Here’s how you might use it:

CREATE TABLE Test2 (
    id INT NOT NULL,
    somecolumn VARCHAR(10)
);

ALTER TABLE Test SWITCH TO Test2;
DROP TABLE Test;

EXEC sp_rename 'Test2', 'Test';
DBCC CHECKIDENT('Test');

Considerations and Alternatives

• Foreign Keys and Constraints: When modifying tables, especially using ALTER TABLE...SWITCH, you may need to drop foreign keys and other constraints before making changes and then reapply them afterward.
• Indexing: The new table’s indexes must match the original table’s exactly for ALTER TABLE...SWITCH to work correctly.
• Sequence Instead of Identity: For SQL Server 2012 and later, using a sequence as a default value can offer more flexibility than traditional identity columns.

Best Practices

• Always back up your database before making significant schema changes.
• Test modifications in a development environment first.
• Consider the impact on existing applications and queries that may rely on the original table structure.

By understanding these methods and their implications, you can effectively modify existing columns to include identity properties, ensuring your database remains flexible and adaptable to changing requirements.