title: SQL Database : table column has relationship to itself
body: <p>Good day mate, I have a question here. I am trying to make a simple enrollment system, so I came up to a "subjects" table. The problem is this subject table has a prerequisite column that the value is another subject from the same table.</p>
  
  <pre><code>E.g
  | subID    | title         | prerequisite   |
  |    345   | math54        | 555            |
  |    555   | physi43       | null           |
  </code></pre>
  
  <p>My question is I am doing it right? In my perspective, it's not because I can't make a foreign key at all. And to make it worst, one subject I have multiple prerequisite value</p>