How to Set a Primary Key Column as Auto Increament in Sql Server

column_name bigint primary key identity

DROP TABLE TEST
GO

create table test
( 
  code varchar(100),
  DESCR varchar(1000)
  CONSTRAINT P_test_code PRIMARY KEY CLUSTERED(code)
)
go


insert into test(code, DESCR)
select 'A', 'AAAA'
UNION ALL
select 'B', 'BBBBB'

go

select *
from	test
go

ALTER TABLE test DROP CONSTRAINT P_test_code
GO

ALTER TABLE test ADD CODE2 INT IDENTITY
GO


ALTER TABLE test
ADD PRIMARY KEY (CODE2)
go

select *
from	test

DECLARE @id INT
SET @id = 0 
UPDATE table SET @id = id = @id + 1