title: Left join weirdness
body: <p>At least with MariaDB v10.x, why truth clauses with left joins doesn't work as I expect when there is <code>NULL</code> return value?</p>
  
  <p>The below works:</p>
  
  <pre><code>SELECT
      u.id
  FROM
      user u
  
      INNER JOIN role r on r.user = u.id
      INNER JOIN customer c ON c.id = r.customer
      LEFT JOIN customer_subclass cs ON cs.customer = c.id
  
  WHERE
      u.status = 'NEW' AND (cs.code != 4 OR cs.code IS NULL)
  </code></pre>
  
  <p>but when I first tried</p>
  
  <pre><code>WHERE
      u.status = 'NEW' AND cs.code != 4
  </code></pre>
  
  <p>it didn't work when <code>cs.code</code> was <code>NULL</code>. Why do I have to specifically test against <code>NULL</code> itself? I would assume <code>NULL != 4</code>?</p>