How to add an identity column to a table with data?

This article demonstrates a step-by step procedure about how an identity column an be added to a table that has data. There are two ways of adding an identity column to a table with existing data:

1. Create a new table with identity, copy data to this new table then drop the existing table followed by renaming the temp table.
2. Create a new column with identity & drop the existing column.

With the second method of implementation the data of the intended identity column cannot be retained, hence we are only interested in the first method. Now lets take a look at how the script for Method 1 can be implemented.

if exists (select 1 from information_schema.tables where table_name = 'DataTable')
  drop table DataTable
GO

Create TABLE DataTable(id int, name varchar(20) )
Insert into DataTable(id, name)
Values(1, 'test 0'),
(4, 'test 1'),
(5, 'test 2'),
(6, 'test 3')
GO

--Create a temp table with an identity column
BEGIN TRANSACTION
GO

CREATE TABLE dbo.Tmp_DataTable
(
  id int NOT NULL IDENTITY (1, 1),
  name varchar(20) NULL
) ON [PRIMARY]
GO

SET IDENTITY_INSERT dbo.Tmp_DataTable ON
GO

IF EXISTS (SELECT 1 FROM dbo.DataTable)
INSERT INTO dbo.Tmp_DataTable (id, name)
SELECT id, name FROM dbo.DataTable WITH (HOLDLOCK TABLOCKX)
GO

SET IDENTITY_INSERT dbo.Tmp_DataTable OFF
GO

DROP TABLE dbo.DataTable

GO

EXECUTE sp_rename N'dbo.Tmp_DataTable', N'DataTable', 'OBJECT'
GO

COMMIT

select * from DataTable

This will be the output of executing the above script:

id name
——– ———-
1 test 0
4 test 1
5 test 2
6 test 3

Now you have a table with data that has an identity column. Notice that the data of an identity column does not have to be contiguous, thought it has to be in ascending order. Caution must be exercised here to drop and recreate all table constraints and relations.

Tip 1: how to reseed an identity column of a table to a new value

–Lets assume the table needs to be reseeded to a value of 100
Declare @newValue int
set @newValue = 100
DBCC CHECKIDENT (‘MyDataTable’, reseed, @newValue – 1)

Tip 2: how to reset the identity column of a table.

Delete from MyDataTable
DBCC CHECKIDENT (‘MyDataTable’, reseed, 0)

Tip 3: how to find the last inserted identity value.

SQL Server provides are three methods of fetching the last insere.ted identity value and all three methods have different scope with in a database operational context. I think Scope_Identity is the most preferred and secure method of fetching the identity value

1. select @@IDENTITY returns the last identity value that was inserted into a table by you are by any other entity in the database scope. If there is no identity insertion @@IDENTITY returns NULL.
2. select SCOPE_IDENTITY() returns the last identity value that was inserted into a table by you or your connection in the database scope. 
3. select IDENT_CURRENT(‘DataTable’) returns the last identity value of a given table.