Here is one that use partitions:

USE AdventureWorks2012;

GO

SELECT SalesOrderID, ProductID, OrderQty

,SUM(OrderQty) OVER(PARTITION BY SalesOrderID) AS Total

,CAST(1. * OrderQty / SUM(OrderQty) OVER(PARTITION BY SalesOrderID)

*100 AS DECIMAL(5,2))AS "Percent by ProductID"

FROM Sales.SalesOrderDetail

WHERE SalesOrderID IN(43659,43664);

GO