It can be done in a single command. You need to set the IDENTITY property for "auto number":

ALTER TABLE MyTable ADD mytableID int NOT NULL IDENTITY (1,1) PRIMARY KEY

More precisely, to set a named table level constraint:

ALTER TABLE MyTable
   ADD MytableID int NOT NULL IDENTITY (1,1),
   CONSTRAINT PK_MyTable PRIMARY KEY CLUSTERED (MyTableID)

See ALTER TABLE and IDENTITY on MSDN

If the table already contains data and you want to change one of the columns to identity:

First create a new table that has the same columns and specify the primary key-kolumn:

create table TempTable
(
    Id int not null identity(1, 1) primary key
    --, Other columns...
)

Then copy all rows from the original table to the new table using a standard insert -statement.

Then drop the original table.

And finally rename TempTable to whatever you want using sp_rename :

http://msdn.microsoft.com/en-us/library/ms188351.aspx

You can also perform this action via SQL Server Management Studio.
Right click on your selected table -> Modify

Right click on the field you want to set as PK --> Set Primary Key

Under Column Properties set "Identity Specification" to Yes, then specify the starting value and increment value.

Then in the future if you want to be able to just script this kind of thing out you can right click on the table you just modified and select

"SCRIPT TABLE AS" --> CREATE TO

so that you can see for yourself the correct syntax to perform this action.

If you have the column it's very easy.

Using the designer, you could set the column as an identity (1,1): right click on the table → design → in part left (right click) → properties → in identity columns, select #column.

Properties:

Identity column:

In SQL Server 2008:

• Right click on table
• Go to design
• Select numeric datatype
• Add Name to the new column
• Make Identity Specification to 'YES'

you can try this...

ALTER TABLE Your_Table
 ADD table_ID int NOT NULL PRIMARY KEY auto_increment;