title: INSERT if the value doesn't already exists
body: <p>In my database, I have <code>tickets_users</code> that affiliate users to a ticket.</p>
  
  <p>I'd like to do an <code>INSERT</code> that checks in <code>tickets_users</code> if there is not already this <code>users_id</code> (for example user 1444) and <code>type = 2 WHERE tickets_id = something</code> (for example ticket number 455); if it already exists do nothing, else <br></p>
  
  <pre><code>INSERT INTO tickets_users (tickets_id, users_id, type) VALUES (455, 1444, 2) 
  WHERE tickets_id = 455
  </code></pre>