title: Complicated MySQL Query on Two Tables
body: <p>I have one table called "offers" with columns "offer_id", "offer" and "hid".
  The primary key is "offer_id".</p>
  
  <p>I have another table called "dates" with columns "hid", "date", "offer_id" and some others.
  The primary key is "date" and "hid". The same value of "offer_id" can exist in many rows (or none at all).</p>
  
  <p>How can I write a mySQL query to find offer_ids that exist in "offers" but not in "dates"?</p>
  
  <p>I tried</p>
  
  <pre><code>SELECT offers.offer_id FROM offers JOIN dates ON offers.offer_id NOT IN dates
  </code></pre>
  
  <p>but that didn't work.</p>