title: GROUP and SUM in Entity Framework
body: <p>I want to select sum of all (paid) prices of an order item for each customer.
  Here is SQL command: </p>
  
  <pre><code>SELECT c.name,SUM(oi.price * oi.count) from customer c
  JOIN order o ON c.id=o.customer_id
  JOIN order_item oi ON o.id=oi.order_id
  JOIN bill b ON b.id=oi.bill_id
  WHERE b.payment_id is NOT null
  GROUP by c.name;
  </code></pre>
  
  <p>I don't know how to do this in EF.
  Example result:  </p>
  
  <pre><code>John Smith  1500,2  
  Allan Babel 202,0  
  Tina Crown  3500,78
  </code></pre>
  
  <p>(comma is used as decimal point..because price is decimal value)</p>