title: Natural join with more than one common attribute in two tables
body: <p>I can understand how natural join works when the two tables have only one common attribute. What if they have two ones? 
  Table 1 have 3 attributes: <strong>A</strong>, <strong>B</strong>, C
  Table 2 has 3 attribute: <strong>A</strong>, <strong>B</strong>, D</p>
  
  <p>First two rows in table 1:</p>
  
  <pre><code>1 2 3
  4 5 6
  </code></pre>
  
  <p>First two rows in table 2:</p>
  
  <pre><code>1 3 4
  8 5 8
  </code></pre>
  
  <p>What is the result of a natural join between the two tables?</p>