title: Issue with unwanted numbers in HTML table
body: <p>I have this issue with my HTML table: It is a table of grades, and I used -1 as a pilot grade that means the student hasn't done the exam yet, therefore he didn't get his grade yet. But I don't want it to show up in the table like this:</p>
  
  <p><img src="https://i.imgur.com/iv5ALde.png" alt=""></p>
  
  <p>I want it to have a blank space there. But SQL won't let me leave it null, and I already did all the corner-cases checking with -1. What should I do to make this disappear?</p>
  
  <p>This is a final project for cs50 course, and I am using one of their distribution code as base. Please ignore the image on top, this is a prototype!</p>