title: Presto query : concat multiple rows into one string
body: <p>I have a table: my_table</p>
  <pre><code>customer_id
  -------------
   2156
   6781
   3145
   1235
   9874
  </code></pre>
  <p>I want the output to be one concat string with comma like: <code>2156, 6781, 3145, 1235, 9874</code>
  So far I export the table and using python to do it. I am wondering could I do it directly in Presto query? Thanks!</p>