DataSet ds = LoadDataSetUsingDataAdapter();
DataTable orders = ds.Tables["Orders"];
DataTable orderDetails = ds.Tables["OrderDetails"];
ds.Relations.Add( "OrderDetails",
orders.Columns["OrderID"],
orderDetails.Columns["OrderID"]);

var query =
from o in orders.AsEnumerable()
where o.Field<DateTime>( "OrderDate" ).Year >= 1998
orderby o.Field<DateTime>( "OrderDate" ) descending
select new { OrderID = o.Field<int>( "OrderID" ),
OrderDate = o.Field<DateTime>( "OrderDate" ),
Amount = o.GetChildRows( "OrderDetails" ).Sum(
od => od.Field<decimal>( "UnitPrice" )
			* od.Field<short>( "Quantity" ) ) };
