How Can I Add an Identity Column to an Existing SQL Server Table with Data?

Adding Identity Columns to Existing SQL Server Tables

Problem: You need to add an identity column to a SQL Server table that already contains data. Directly altering an existing column to become an identity is not supported.

Solutions: Two methods exist to achieve this:

Method 1: Create a New Table

This method preserves existing data. A new table is created with the identity column, data is copied, and the old table is replaced.

T-SQL:

CREATE TABLE dbo.Tmp_Names (
    Id INT NOT NULL IDENTITY(1, 1),
    Name VARCHAR(50) NULL
) ON [PRIMARY]
GO

SET IDENTITY_INSERT dbo.Tmp_Names ON
GO

IF EXISTS (SELECT * FROM dbo.Names)
    INSERT INTO dbo.Tmp_Names (Id, Name)
    SELECT Id, Name FROM dbo.Names TABLOCKX
GO

SET IDENTITY_INSERT dbo.Tmp_Names OFF
GO

DROP TABLE dbo.Names
GO

EXEC sp_rename 'Tmp_Names', 'Names'

Method 2: Add a New Identity Column

This approach adds a new identity column, removes the old primary key, and renames the new column. Note: Existing data in the original primary key column will not be preserved.

T-SQL:

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

ALTER TABLE Names DROP COLUMN ID
GO

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

Important Considerations:

• Backups: Always back up your data before performing schema changes.
• Transactions: Consider wrapping these operations in a transaction for atomicity.
• Testing: Thoroughly test these changes in a development or staging environment before applying them to production.
• Data Loss: Method 2 results in data loss in the original primary key column. Choose the appropriate method based on your data requirements.

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