title: Concatenate Rows with Matching IDs
body: <p>I have multiple rows of purchase details. Each purchase has a client ID. For presentation purposes I need to merge purchases with a similar client ID into a single cell so I can use a VLOOKUP to display this in another table that has client information. Any ideas?</p>
  
  <p>In the example below, I'd like cell C2 to contain "1, 2", cell C3 to contain "3" and cell C4 to be empty (bill has made no purchases).</p>
  
  <pre><code>     A       B      C
  1 client_id name  purchase_ids
  2         1 jim
  3         2 bob
  4         3 bill
  
  
  purchase_id purchase_client_id amount
            1                  1    100
            2                  1    500
            3                  2     50
  </code></pre>