You can add an identity column, which automatically assigns a value:
-- Create test table without primary key
CREATE TABLE dbo.Person
(
Name varchar(30) null
)
INSERT INTO dbo.Person (Name)
SELECT 'Abe One'
UNION ALL
SELECT 'Bee Two'
GO
-- Add new identity column
ALTER TABLE dbo.Person ADD
ID int NOT NULL IDENTITY (1, 1)
GO
-- Make it the primary key
ALTER TABLE dbo.Person ADD CONSTRAINT
PK_Person PRIMARY KEY CLUSTERED
(
ID
)
GO
Or you can use the Management Studio visual designer. Right click on the table name, then select 'Design' from the menu. For more details see: http://technet.microsoft.com/en-us/library/ms190634(v=sql.105).aspx