title: Two select statements in one query
body: <p>I need to get the users age by his ID. Easy.
  The problem is, at the first time I don't know their IDs, the only thing I know is that it is in a specific table, let's name it "second".</p>
  
  <pre><code>SELECT `age` FROM `users` WHERE `userid`=(SELECT `id` FROM `second`)
  </code></pre>
  
  <p>How can I do that?</p>