title: SQL: Oracle avg over group
body: <p>I would like to show my problem on example.
  So, I have students, they are studying in different schools and I would like to count average by schools and then count total average.</p>
  
  <p>Example:</p>
  
  <p><a href="https://i.stack.imgur.com/aFTwL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/aFTwL.png" alt="example"></a></p>
  
  <p>So, how could i get result like this one?</p>
  
  <p>Thanks in advance!</p>
  
  <p>One more option - it is really much more comfortable for me to do this in one query.</p>