title: Default values for missing fields in a left join?
body: <p>In Pig, when I do left join and a row does not have row, the values are <code>NULL</code>:</p>
  
  <pre><code>c = join a by ($0) left, b by ($0);
  </code></pre>
  
  <p>if</p>
  
  <pre><code>a=((1,10),(2,20))
  b=((1,30))
  </code></pre>
  
  <p>then </p>
  
  <pre><code>c=((1,10,30),(2,20,NULL))
  </code></pre>
  
  <p>I want to use a default value (say, <code>-1</code>) instead of <code>NULL</code> so that</p>
  
  <pre><code>c=((1,10,30),(2,20,-1))
  </code></pre>
  
  <p>How do I do that?</p>
  
  <p>If that is impossible, how do I change the 3rd column of <code>c</code> to have the default value instead of <code>NULL</code>?</p>