title: Google Spreadsheets: How do you concat strings in an aggregation function
body: <p>Say I have a table:</p>
  
  <pre><code>A, 1
  B, 1
  C, 2
  D, 1
  E, 2
  </code></pre>
  
  <p>How do I view the table grouping by the 2nd column and aggregating by the first with a comma separated concat function ie:</p>
  
  <pre><code>1, "A,B,D"
  2, "C,E"
  </code></pre>
  
  <p>In both defining a pivot table and using the QUERY syntax, it seems that the only aggregation functions available are numerical aggregations like MIN, MAX, SUM, etc. Can I define my own aggregation function?</p>