title: return all ids where the count in table1 is equal to the count in table2
body: <p>I am joining two tables by the field of ids. Let's call these tables p and k. <code>k</code> is a superset of <code>p</code> in this situation; all things in p are in k. The same id may show up multiple times or have multiple entries.</p>
  
  <p>The idea, however, is I want to see if the count of times the id shows up in <code>p</code> is exactly the same as the number of times it shows up in <code>k</code>. If it is, those are the ids I want to show in my query.</p>
  
  <p>This is all I really have to start.</p>
  
  <pre><code>SELECT
    p.id
  FROM
    all_files k,
    specific_files p
  WHERE
    p.id = k.id
  </code></pre>
  
  <p>How would this be done?</p>