title: Database MN relationship
body: <p>I'm taking a class on database management systems (absolute beginner) and I'm working on a  database for a very simple blog system. </p>
  
  <p>I have a question regarding one M:N relationship between blog posts and categories where the posts belong (one blog posts can be in several categories.)</p>
  
  <p>The part of the scheme looks like this:
  <a href="http://creo-preview.xf.cz/blog_posts.jpg" rel="nofollow">Scheme</a></p>
  
  <p>I know that somehow this scheme allows to add a blog post that doesn't belong to any category. However, I don't know why that is. Could someone please explain this to me?</p>
  
  <p>Thanks.</p>