Ensuring uniqueness

The first question that naturally comes to mind is: what is uniqueness? In a traditional RDBMS database design, you must provide a mechanism whereby each database table row is different from every other row. If you had two RDMBS table rows, X and Y, which were identical, it would be impossible to guarantee access to X or to Y. Further, it would be a pointless waste of space to maintain duplicate data in such a manner. Accordingly, in the RDBMS world, when designing your database, you need to identify one or more database columns as part of the primary key.