Using a Subquery to Update a Single Column

You can update the column specified in the SET clause with the value that a subquery returns. 
UPDATE orders
   SET ship_charge =
      (SELECT SUM(total_price) * .07 FROM items
         WHERE orders.order_num = items.order_num)
   WHERE orders.order_num = 1001;

If you are updating a supertable in a table hierarchy, the SET clause cannot include a subquery that references a subtable. If you are updating a subtable in a table hierarchy, a subquery in the SET clause can reference the supertable if it references only the supertable. That is, the subquery must use the SELECT ...; FROM ONLY (supertable) syntax.