The sum()-function returns the sum of its argument. This function requires an XPath query as argument. The query must specify an attribute that has a numeric type.
This query returns the sum of the total prices of all placed orders.
sum(//Sales.Order[Sales.Customer_Order/Sales.Customer/Name = 'Jansen']/TotalPrice)
This query returns the sum of the total prices of all orders placed by a customer named Jansen.