title: compare values in a column and find if there is an increment
body: <p>I have a question. 
  I am trying to find if there is an increment or decrement of sales by comparing the sales of 4 months.</p>
  
  <p>How do we do this in sql.</p>
  
  <pre><code>Months  Sales
  ----    -----
  jan       30
  feb       10
  mar       60
  april     20
  april     20
  </code></pre>