Want to add 3rd Column to Gridview showing total units of products sold??

hi,
i have a Data Set with 2 table adaptors - Products and Sales linked to SQL database.
Products Table
Product_Id (int)
ProductName
Sales Table
SalesId (int)
DateSold
UnitsSold
Product_Id (Foreign Key with ProductsTable)

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On the aspx page I have a gridview attached to an objectDataSource.
This is binded to the Products Table Adaptor in The DAL and returns a GETDATA() Method
thats shows 2 columns   "ProductId" and "ProductName"
I want to add a 3rd column to the Gridview "Units Sold"  that shows the total
number of products sold by each ProductId.  i am assuming this will be a "Template Column"
I read somwhere I can do this by Adding a Class in the App_Code Folder but
would lik ...