title: How to emulate SQLs rank functions in R?
body: <p>What is the R equivalent of rank functions like the Oracle <code>ROW_NUMBER()</code>, <code>RANK()</code>, or <code>DENSE_RANK()</code> ("assign integer values to the rows depending on their order"; see <a href="http://www.orafaq.com/node/55" rel="nofollow noreferrer">http://www.orafaq.com/node/55</a>)? </p>
  
  <p>I agree that the functionality of each function can potentially be achieved in an ad-hoc fashion. But my main concern is the performance. It would be good to avoid using join or indexing access, for the sake of memory and speed.</p>