title: convert letter to number in sql
body: <p>I have a student table, one of its column contains the grade obtained by student(eg, A,B,C...). I need to assign a number to every grade (eg, A->10, B->8, C->6 ...)</p>
  
  <p>I am new to sql and I am not getting any idea on how to do it.
  I tried one method which I found online:</p>
  
  <pre><code>select letter_grade, case when 'A' then 4 end as lettergrade from students;
  </code></pre>
  
  <p>But its not working</p>