title: MySQL not null in left join condition
body: <p>My query looks like:</p>
  <pre class="lang-sql prettyprint-override"><code>   SELECT * 
       FROM users U 
  LEFT JOIN posts P ON P.userId = U.id AND P.userId IS NOT NULL;
  </code></pre>
  <p>Why the query also return result where <code>userId</code> is <code>null</code> ?</p>
  <p>I know that for my needs I can use <code>INNER JOIN</code> to get only posts related to user but is so strange that <code>LEFT JOIN</code> support multiple conditions, but not work with NOT NULL conditions.</p>