title: mySQL query: order by group points by sum of join member points
body: <p>I got 2 tables which are linked.
  GROUPS can have multiple USERS and USERS can be in multiple GROUPS.</p>
  
  <pre><code>GROUPS
  |id| name      |
  ----------------
  |1|  Koalas    | 
  |2|  Grizzlies  | 
  |3|  Hornets    |
  ----------------
  
  USERS
  |id| firstName |  points  |
  ----------------
  |1|    Bob     |  2
  |2|    Hans    |  4
  |3|    Jerome  |  1
  |4|    Katy    |  6
  ----------------
  
  GROUP_USER
  |id| group_id  | user_id  |
  --------------------------
  |1|  1         |  2
  |2|  1         |  4
  |3|  2         |  1
  |4|  2         |  2
  |5|  3         |  3
  |6|  3         |  4
  ----------------
  </code></pre>
  
  <p>Now I want to rank the groups by the points of their members.
  Result:</p>
  
  <pre><code>GROUP_USER
  |rank| group_name  | user_points  |
  -----------------------------------
  |1   |  Koalas     |  10
  |2   |  Hornets    |  7 
  |3   |  Grizzlies  |  6
  </code></pre>
  
  <p>Don´t really know how to start.</p>