title: SQL Query to find missing rows between two related tables
body: <p>I have two tables:</p>
  
  <h2>Table A</h2>
  
  <ul>
  <li>ID</li>
  <li>ABC_ID</li>
  <li>VAL</li>
  </ul>
  
  <h2>Table B</h2>
  
  <ul>
  <li>ID</li>
  <li>ABC_ID</li>
  <li>VAL</li>
  </ul>
  
  <p>These two tables are directly related to each other through the <code>ABC_ID</code> column.</p>
  
  <p>I want to find all the <code>VAL</code> column values in table A which are <em>not</em> present in table B for the same ABC_ID. </p>