title: Return 0 if field is null in MySQL
body: <p>In MySQL, is there a way to set the "total" fields to zero if they are NULL?</p>
  
  <p>Here is what I have:</p>
  
  <pre><code>SELECT uo.order_id, uo.order_total, uo.order_status,
              (SELECT SUM(uop.price * uop.qty) 
               FROM uc_order_products uop 
               WHERE uo.order_id = uop.order_id
              ) AS products_subtotal,
              (SELECT SUM(upr.amount) 
               FROM uc_payment_receipts upr 
               WHERE uo.order_id = upr.order_id
              ) AS payment_received,
              (SELECT SUM(uoli.amount) 
               FROM uc_order_line_items uoli 
               WHERE uo.order_id = uoli.order_id
              ) AS line_item_subtotal
              FROM uc_orders uo
              WHERE uo.order_status NOT IN ("future", "canceled")
              AND uo.uid = 4172;
  </code></pre>
  
  <p>The data comes out fine, except the NULL fields should be <code>0</code>.</p>
  
  <p>How can I return 0 for NULL in MySQL?</p>