title: Filter after a foreach statement in pig
body: <p>So i have table with one column, U and has column name u_id.</p>
  
  <pre><code>filter_out = filter A BY s_id == (FOREACH u GENERATE u_id);
  </code></pre>
  
  <p>I am basically trying to filter A table by match of every row in u table. 
  So essentially if s_id from A(table1) matches u_id from 2nd tables, filter it out</p>
  
  <p>I keep getting  mismatched input 'u' <code>expecting LEFT_PAREN</code> error</p>
  
  <p>-------------2nd approach----------------</p>
  
  <p>And also have tried converting u to a tuple</p>
  
  <pre><code>totuple = FOREACH u GENERATE TOTUPLE (u_id);
  
  filter_out = filter A BY s_id in (totuple);
  </code></pre>
  
  <p>and error <code>A column needs to be projected from a relation for it to be used as a scalar
  </code></p>