How Can I Add an Identity Column to an Existing Database Table?

Modifying Existing Tables to Include Identity Columns

Many database tasks require adding identity columns to existing tables. A frequent question is whether to modify an existing column or create a new one.

Why Direct Conversion Isn't Possible

Directly transforming an existing column into an identity column isn't feasible. Identity columns generate sequential values starting from a defined seed value. Modifying an existing column would disrupt data integrity.

Strategies for Adding an Identity Column

Two main methods exist for adding an identity column:

1. Creating a New Table with Identity

This involves:

• Generating a script to transfer data from the original table to a new table, which includes the identity column.
• Removing the original table.
• Renaming the new table to match the original table's name.
• This method preserves the original data values (although they might not be sequential).

2. Adding a New Identity Column

This approach consists of:

• Adding a new column with the identity property.
• Removing the original column (or migrating data to the new column).
• Renaming the new column to match the original column's name.
• This method does not retain the original data values in the identity column; the identity column will generate new sequential values.

Illustrative SQL Queries

Method 1: New Table Creation

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

INSERT INTO New_Table (Id, Name)
SELECT Id, Name
FROM Original_Table;

DROP TABLE Original_Table;

EXEC sp_rename 'New_Table', 'Original_Table';

Method 2: Adding a New Column

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

ALTER TABLE Original_Table
DROP COLUMN Id;

EXEC sp_rename 'Original_Table.Id_new', 'Id', 'COLUMN';

Important Considerations

• When creating a new table with an identity column, use the NOT NULL constraint. Set IDENTITY_INSERT to ON before inserting data to maintain existing values (if desired).
• The identity column will automatically generate sequential numbers, replacing any pre-existing values in the column.

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