title: Select multiple rows with matching id(s)
body: <p>I tried for hours and read many posts but I still can't figure out how to handle this request:</p>
  
  <p>I have a table like this:</p>
  
  <pre><code>+------+------+
  |roleid|comid |
  +------+------+
  |11    | A    |
  +------+------+
  |12    | A    |
  +------+------+
  |11    |B     |
  +------+------+
  |12    |B     |
  +------+------+
  |13    |B     |
  +------+------+
  </code></pre>
  
  <p>I would like to select the roleid that occurs more than once with the different comid.</p>
  
  <p>I wanna output should be something like:</p>
  
  <pre><code>+------+------+
  |roleid|comid |
  +------+------+
  |11    |A,B   |
  +------+------+
  |12    |A,B   |
  +------+------+
  |13    |B     |
  +------+------+
  </code></pre>