title: Concat single column fields using GROUP BY
body: <p>Is there any way to combine/concat the fields within one column by grouping them.
  Eg:</p>
  
  <pre><code>col1   col2
  1     aa
  1     bb
  1     cc
  2     dd
  2     ee
  </code></pre>
  
  <p>I want to query something like :</p>
  
  <pre><code>select col1, concat(col2) from tableName group by col1;
  </code></pre>
  
  <p>Output should be :</p>
  
  <pre><code>1    aa,bb,cc
  2    dd,ee
  </code></pre>
  
  <p>Is there any function in hive to do this ?</p>