title: Basic SQL user trying to understand advanced SUM functionality
body: <p>I'm going through the "Select within Select" tutorial on SQLzoo.net. It occurred to me that I'd like to know the total area of each continent. The table <code>world</code> has 5 columns. </p>
  
  <ol>
  <li><code>name</code> is country name</li>
  <li><code>continent</code> is for where the country belongs</li>
  <li><code>area</code> is for the total area of each country</li>
  <li><code>population</code> is present but not needed for this</li>
  <li><code>gdp</code> is present but not needed for this</li>
  </ol>
  
  <p>I want my result to be two columns(<code>Continent</code> and <code>Area</code>). I want Area to be the sum of each country's <code>area</code> located on the same <code>Continent</code>.</p>
  
  <p>How can I separate out the countries by <code>continent</code> and then add them to the appropriate <code>continent</code> in the data returned?</p>