title: How to change column name and value depending on condition?
body: <p>I want to change column name and value depending on condition.
  My table is..</p>
  
  <pre><code>Roll    Name    Mark
  3   Chaity  87
  1   Anis    75
  4   Unknown 30
  2   Badol   0
  </code></pre>
  
  <p>And I want to get like this</p>
  
  <pre><code>Roll    Name    Grade
  3   Chaity  A+
  1   Anis    A
  4   Unknown F
  2   Badol   F
  </code></pre>
  
  <p>Where Mark and Grade mapping is as follows:</p>
  
  <pre><code>0 to 60 is F
  61 to 79 is A
  80 and above is A+
  </code></pre>