title: add an identity column to existing table as the primary key and change order
body: <p>I have a table that has over 7 million records in it. The table does not have a primary key. I would like to add a new identity column and set this as the primary key. I tried adding the column using SSMS, then I set it as the primary key. I called this new column Id. </p>
  
  <p>This almost worked, however I wanted to change the default order of the table to be based another column, for instance a date time column in descending order. Is this possible? Perhaps I need to use a temp table and the <code>ROW_NUMBER()</code> function. </p>
  
  <p>However, I am not very good at SQL. Can someone help? </p>
  
  <p>I also need to have a rollback script so I can get back to the original table. </p>