title: sum the marks and rank them by number
body: <p>I have table students table with these fields </p>
  
  <pre><code>student  continent        marks
  john      Asia             500
  jack      south America    600
  Amy       Africa           450
  Olive     Africa           600
  Jay       Africa           755
  Mackey     Asia            699
  James     Europe           823
  </code></pre>
  
  <p>I've tired to sum the marks by below code to print continent and total marks -</p>
  
  <pre><code>SELECT  continent, sum(marks) as Totalmarks 
  from students 
  group by continent 
  order by Totalmarks desc;
  </code></pre>
  
  <p>What should I do to rank them and print in below desired output -</p>
  
  <pre><code>continent  Totalmarks  Rank
  </code></pre>