title: How do I select a row from one table where the value row does not exist in another table?
body: <p>Let's say I have two identical tables, A and B, with the row "x".</p>
  
  <p>I want to select all elements in A, where the value of x in A is not in any value of x of B.</p>
  
  <p>How do I do that?</p>