title: Why is the column reference ambiguous in INNER JOIN, but not in NATURAL JOIN?
body: <p>I'm wondering why PostgreSQL produces an ambiguous reference error on this statement:</p>
  
  <pre><code>SELECT c from A JOIN B ON A.c = B.c; -- ERROR:  column reference is ambiguous
  </code></pre>
  
  <p>but not this:</p>
  
  <pre><code>SELECT c FROM A NATURAL JOIN B; -- OK
  </code></pre>
  
  <p>In both cases <code>A.c</code> must be identical to <code>B.c</code>.</p>