title: How to (MySQL) multiply columns and then the sum rows?
body: <p>I've got this table:</p>
  
  <pre><code>id    |   payment_id   | quantity  |  unit cost
  1           633             1            750  
  2           633             1            750 
  3           632             2            750 
  4           632             2            750 
  </code></pre>
  
  <p>What I need is :</p>
  
  <pre><code>payemnt_id  |   total
      633           1500
      632           3000 
  </code></pre>
  
  <p>You can also think of this as Countries and then you are trying to find the total for each Country.  I was sure there were some tutorials like this but could not find by STFW.</p>