title: Select rows with equal values
body: <p>How to do in Laravel?</p>
  
  <p>1) get from table rows where <code>user_id</code> has the equal values?</p>
  
  <p>2) return sum of <code>some_amount</code> values from this selected rows</p>
  
  <pre><code>Table:
   - id;
   - user_id;
   - some_amount;
  </code></pre>
  
  <p>Table has bunch of certificates with some amount of money that belongs to different users. I need to find all certificates that belongs to one user (one user can have few certificates) and count how much money he have from all his certificates</p>