title: MySQL - LEFT JOIN and select rows from table not in another
body: <p>Ok, so here is an existing query I have that does what I need.</p>
  
  <pre><code>SELECT C1.id AS id, C1.title AS title, C1.instructor_id AS instructor_id, 
    C1.description AS description, C1.date AS date, C1.starttime AS starttime, 
    C1.endtime AS endtime, C1.training_id AS training_id, C2.name AS name, 
    C2.id AS instructors_id 
  FROM trainings C1 
  LEFT JOIN instructors C2 ON C1.instructor_id = C2.id
  </code></pre>
  
  <p>However, I need to add something to it. I have two tables I need to compare in addition to what I have.</p>
  
  <p>Table 'trainings' has a list of training sessions, indexed by 'id'. I also have another table, 'registrations', that holds info about what users registered for what training sessions with 3 columns: 'id' - index, 'user_id' - the ID of the user that registered for a training, and 'course_id' - the ID of the training session.</p>
  
  <p>What I need to do in addition to my existing query, is select ONLY the training sessions that do not have a row in 'registrations' for that user already.</p>
  
  <p>I hope this makes sense. Thank you for who can help. I have tried for hours. The query is just too large for me to keep organized and to think properly about how to do it.</p>