title: SQL: Returning unassigned rows
body: <pre><code>assignments:
  id | prospectid
  1  | 1
  2  | 2
  3  | 5
  
  prospects:
  id | name
  1  | purple
  2  | red
  3  | blue
  4  | orange
  5  | green
  </code></pre>
  
  <p>I'm wanting to return a list of prospects where prospect.id does not exist under the prospectid column within assignments. In a nutshell, I want to return unassigned prospects.</p>
  
  <p>So from the above example, it ought to return:</p>
  
  <pre><code>prospects:
  3 | blue
  4 | orange
  </code></pre>
  
  <p>My failed SQL:</p>
  
  <pre><code>SELECT * FROM prospects
  WHERE prospects.id != `assignments`.prospectid
  </code></pre>