什么是DataRelation？

DataRelation customerOrdersRelation = customerOrders.Relations.Add("CustOrders",

customerOrders.Tables["Customers"].Columns["CustomerID"],

customerOrders.Tables["Orders"].Columns["CustomerID"]);

设置Relation有什么影响？

            foreach (DataRow custRow in customerOrders.Tables["Customers"].Rows)
            {
              ……

                foreach (DataRow orderRow in custRow.GetChildRows(customerOrdersRelation))
                {
                  ……
                }

GetChildRows得到的orderRow到底能获取什么东西？