List the most popular item being ordered
To understand the logic I suggest you read this.

http://www.w3resource.com/sql/aggregate-functions/count-with-group-by.php

Try this:

SELECT Item#, SUM(Qty) AS SoldQty
FROM ORDER_ITEM
GROUP BY Item#
ORDER BY SoldQty --You can use 'Desc' depending on how you want to see the data
  ;