title: how to write this sql
body: <p>I'm using mysql</p>
  
  <p>there are two tables</p>
  
  <p><strong>vote</strong></p>
  
  <pre><code>app_id user_id
  2      1
  3      1
  2      2
  </code></pre>
  
  <p><strong>app</strong></p>
  
  <pre><code>id title content
  1  xx    xxxx   
  2  yy    yyyy
  3  zz    zzzz 
  </code></pre>
  
  <p>I want to sort app table by the number of user's vote . in this example , the result should be</p>
  
  <pre><code>id title content
  2  yy    yyyy
  3  zz    zzzz
  1  xx    xxxx
  </code></pre>
  
  <p>any suggestion?</p>