title: GROUP BY to combine/concat a column
body: <p>I have a table as follow:</p>
  
  <pre><code>ID  User  Activity  PageURL  
   1  Me    act1      ab     
   2  Me    act1      cd     
   3  You   act2      xy     
   4  You   act2      st
  </code></pre>
  
  <p>I want to group by User and Activity such that I end up with something like:</p>
  
  <pre><code>User  Activity  PageURL  
  Me    act1      ab, cd     
  You   act2      xy, st
  </code></pre>
  
  <p>As you can see, the column PageURL is combined together separated by a comma based on the group by.</p>
  
  <p>Would really appreciate any pointers and advice.</p>