title: Find the countries with a GDP greater than a group of countries
body: <p>I'm attempting question 5 of SQLZoo's <a href="http://sqlzoo.net/wiki/SELECT_within_SELECT_Tutorial" rel="noreferrer">SELECT within SELECT tutorial</a></p>
  
  <blockquote>
    <p>Which countries have a GDP greater than any country in Europe? [Give the name only.]</p>
  </blockquote>
  
  <p>This is my solution, which is incorrect but I don't understand why.</p>
  
  <pre><code>SELECT name  
    FROM world  
   WHERE gdp &gt; ALL (SELECT gdp FROM world WHERE continent ='Europe')
  </code></pre>
  
  <p>What am I doing incorrectly?</p>