title: List ids in group query
body: <p>I want to return a row for each cluster of data that has a unique <code>amount</code>, <code>operation</code>, <code>months</code>, and <code>fee</code> for a given id.</p>
  
  <p>Table is as follows:</p>
  
  <p><img src="https://i.stack.imgur.com/gKwWd.png" alt="enter image description here"></p>
  
  <p>I can almost get what I want with</p>
  
  <pre><code>SELECT amount, operation, months, fee, plan_id
  FROM promo_discounts
  WHERE promo_id = 1
  GROUP BY amount, operation, months, fee
  </code></pre>
  
  <p>Which will get:</p>
  
  <p><img src="https://i.stack.imgur.com/6Cumh.png" alt="enter image description here"></p>
  
  <p>But notice it only returns one <code>plan_id</code> when I want it to return something like <code>1, 2, 3</code></p>
  
  <p>Is this possible?</p>