How to Make an Existing Column an Identity Column in SQL Server?

Transforming an Existing Column into an Identity Column in SQL Server

Modifying a table's primary key to an identity column, particularly when the table already contains significant data, requires careful consideration. This guide outlines the best practices for this process.

SQL Solutions for Adding Identity Properties:

Directly altering an existing column to become an identity column isn't feasible in SQL Server. However, two effective workarounds exist:

1. Construct a New Table with Identity:

   CREATE TABLE dbo.Tmp_Names
   (
     Id int NOT NULL IDENTITY(1, 1),
     Name varchar(50) NULL
   )

   Populate the new table with data from the original table, preserving existing data. Finally, replace the old table with the new one.

2. Introduce a New Identity Column:

   ALTER TABLE Names
   ADD Id_new Int IDENTITY(1, 1)

   Remove the original column and rename the new identity column to match the old primary key.

Important Notes:

• The "New Table" method allows preservation of existing data values.
• The "New Column" method does not preserve existing data values in the new identity column.
• For in-depth discussions and further assistance, consult the Microsoft SQL Server Forum's thread on altering columns to identity properties.

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