title: Convert columns data into rows in PostgreSQL
body: <p>I have data in the following format.</p>
  
  <pre><code>order_no    rate       jan        feb       mar     ....
   1           1200                  2         4
   2           1000      1                     5
   3           2400      14          3          
  </code></pre>
  
  <p>Now I want to transpose this table to get the following output.</p>
  
  <pre><code>order_no    rate       month     unit
   1           1200       feb       2 
   1           1200       mar       4 
   2           1000       jan       1
   2           2400       mar       5  and so on..
  </code></pre>
  
  <p>How can I do this?          </p>