title: How to convert date into corresponding days
body: <p>I have a problem about a query.I don't know how to convert date into corresponding days.I have this query </p>
  
  <pre><code>SELECT A.DELIVERY_DATE,B.PO_DATE
   FROM t_mms_po_items A,t_mms_purchase_order B
   WHERE A.PO_ID = B.PO_ID AND A.ITEM_ID =165104
  </code></pre>
  
  <p>when I run this query it shows </p>
  
  <pre><code>`  DELIVERY_DATE  PO_DATE
   1  22-Oct-14    19-Aug-11
   2  11-Oct-14    12-Apr-11
   3  11-Oct-14    21-Apr-14
   4  22-Oct-14    22-Aug-14
  </code></pre>
  
  <p>But I want this in total days, such as 63 days, 90 days or something like that.What should I do? How can I convert date into days?Please help me to sort out.</p>