title: Compare aggregated value for groups to the aggregated value for the entire table
body: <p>Let's say I have one table that has E_name, DepartmentName, Salary</p>
  
  <pre><code>E_name  DepartmentName  Salary
  AAA     D1              1000
  BBB     D1              1400
  CCC     D2              1320
  DDD     D2              1100
  EEE     D3              900
  FFF     D3              1200
  </code></pre>
  
  <p>how to show DepartmentName where their employees' average salary is bigger than the average of all employees</p>