How set Identity(1,1) on created table

Question

I am new SQL world I want to neither rebuild the whole table nor add a new column.

Answer 1

//Drop the column

Alter TABLE [dbo].[t_name]
    drop column [columnname]

//create a new column with identity and with previous column name

Alter TABLE [dbo].[t_name]

add  [columnname] [int] IDENTITY(1,1) NOT NULL

Answer 2

if you want to alter existing table column you can try this: replace Tmp_example1 with tmp_yourtablename replace example1 with yourtablename replace eid with yourcolumnname

BEGIN TRANSACTION
SET QUOTED_IDENTIFIER ON
SET ARITHABORT ON
SET NUMERIC_ROUNDABORT OFF
SET CONCAT_NULL_YIELDS_NULL ON
SET ANSI_NULLS ON
SET ANSI_PADDING ON
SET ANSI_WARNINGS ON
COMMIT
BEGIN TRANSACTION
GO
CREATE TABLE dbo.Tmp_example1
(
eid INT NOT NULL IDENTITY (1, 1)
) ON [PRIMARY] GO
SET IDENTITY_INSERT dbo.Tmp_example1 ON
GO
IF EXISTS(SELECT * FROM dbo.example1)
EXEC('INSERT INTO dbo.Tmp_example1 (eid)
SELECT eid FROM dbo.example1 WITH (HOLDLOCK TABLOCKX)')
GO
SET IDENTITY_INSERT dbo.Tmp_example1 OFF
GO
DROP TABLE dbo.example1
GO
EXECUTE sp_rename N'dbo.Tmp_example1', N'example1', 'OBJECT'
GO
COMMIT