title: Find unique and overlapping subscribers from list that also uses timestamped actions to track status
body: <p>I have one table that tracks all subscribers to various lists. It looks like: list_id, user_id, action_date, action. Users subscribe to a list ID and we capture that with a time stamp and action=1. If they unsubscribe we timestamp a new entry and capture action=0.</p>
  
  <p>We launched a few new subscriptions recently and I'm trying to figure out:</p>
  
  <ol>
  <li>How many users are subscribed to list_id A and list_id B</li>
  <li>How many users are subscribed to just list_id A and not list_id B (and vice versa)</li>
  </ol>
  
  <p>I am still new to SQL so throwing in the wrinkle where the most recent action for each user_id must be the action of 1 is throwing me off.</p>
  
  <p>Can anyone help me?</p>