How do I add auto_increment to a column in SQL Server 2008

Question

I am using SQL Server 2008 and a primary key of a database table I am using is not an IDENTITY column (not sure why). I need to change that.

I am in SQL Server Management Studio in design view, under column properties and for some reason I can't change the identity specifications to Yes.

Is there something that I am missing.. I am new to SQL Server - any ideas on what I am missing??

Here is the create table

CREATE TABLE [dbo].[AR_Transactions](
       [Trans_ID] [bigint] NOT NULL,
       [DateTime] [datetime] NOT NULL,
       [Cashier_ID] [nvarchar](50) NULL,
       [CustNum] [nvarchar](12) NOT NULL,
       [Trans_Type] [nvarchar](2) NOT NULL,
       [Prev_Cust_Balance] [money] NULL,
       [Prev_Inv_Balance] [money] NULL,
       [Trans_Amount] [money] NOT NULL,
       [Payment_Method] [nvarchar](4) NULL,
       [Payment_Info] [nvarchar](20) NULL,
       [Description] [nvarchar](38) NULL,
       [Invoice_Number] [bigint] NOT NULL,
       [Store_ID] [nvarchar](10) NOT NULL,
       [Dirty] [bit] NOT NULL,
       [Station_ID] [nvarchar](5) NULL,
       [Payment_Type] [smallint] NULL,

CONSTRAINT [pkAR_Transactions] 
       PRIMARY KEY CLUSTERED([Store_ID] ASC, [Trans_ID] ASC)
           WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, 
                 ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]

ALTER TABLE [dbo].[AR_Transactions] 
    ADD CONSTRAINT [DF_AR_Transactions_Trans_ID_AR_Transactions] 
    DEFAULT ((0)) FOR [Trans_ID]

ALTER TABLE [dbo].[AR_Transactions] 
    ADD CONSTRAINT [DF_AR_Transactions_Invoice_Number_AR_Transactions] 
    DEFAULT ((0)) FOR [Invoice_Number]

Here is the query that I need to run... its a complete hack to try to auto-increment my inserts myself

BEGIN TRANSACTION 

INSERT INTO 
        [cresql].[dbo].[AR_Transactions](Trans_ID, DateTime , Dirty, Store_ID, Trans_Type,  
            Cashier_ID, CustNum, Trans_Amount, Prev_Cust_Balance) 
        SELECT  
            (SELECT MAX(Trans_ID ) + 1 FROM [cresql].[dbo].[AR_Transactions]), 
            DATEADD(MINUTE, -30, Getdate()), 1, 1001, 'C', 100199, CustNum,
            -Acct_Balance, Acct_Balance 
    FROM  [cresql].[dbo].[Customer] 
        WHERE Acct_Balance <> 0  

UPDATE [cresql].[dbo].[Customer] 
    SET Acct_Balance = 0 
WHERE Acct_Balance <> 0  

COMMIT TRANSACTION

Answer 1

To illustrate Martin's point:

And PS: - as Mikael Eriksson rightfully mentions (and documents nicely), this Identity Specification remains grayed out as long as that column you're working on has a default constraint.

Answer 2

You need to expand the "Identity Specification" node to change it via the (Is Identity) property.

This will rebuild the table so you might also need to go into Tools -> Options -> Designers -> Prevent saving changes that require table re-creation.

This can be an extremely time consuming operation on large tables as well as entailing a lot of logging and locking. To perform this operation on a large table see my answer here.