<Sales>
  <Sale>
    <EmployeeID>279</EmployeeID>
    <OrderID>12345</OrderID>
    <ProductID>776</ProductID>
   <Sale>
  <Sale>
    <EmployeeID>279</EmployeeID>
    <OrderID>1277</OrderID>
    <ProductID>776</ProductID>
   <Sale>
  <Sale>
    <EmployeeID>279</EmployeeID>
    <OrderID>1277</OrderID>
    <ProductID>778</ProductID>
   <Sale>
  <Sales>

<Sales>
  <Sale>
    <EmployeeID>279</EmployeeID>
               <OrderID>12345</OrderID>
                       <ProductID>776</ProductID>
               <OrderID>1277</OrderID>
                           <ProductID>776</ProductID>
                           <ProductID>778</ProductID>
   <Sale>
  <Sales>
    

  var Employees = from Sale in inputXML.Root.Elements("Sale")
                            group Sale by new { EmpId = Sale.Element("EmployeeID").Value } into g
                            orderby g.Key.EmpId
                            select new XElement("EmployeeID", new XAttribute("ID", g.Key.EmpId));

   var Employees = from EmployeeSalesDetail in inputXML.Root.Elements("EmployeeSalesDetail")
                            group EmployeeSalesDetail by new { EmpId = EmployeeSalesDetail.Element("EmployeeID").Value, NatIDNum = EmployeeSalesDetail.Element("NationalIDNumber").Value } into g
                            orderby g.Key.EmpId
                            select new XElement("Employee", new XAttribute("ID", g.Key.EmpId), new XAttribute("NatIDNum", g.Key.NatIDNum),
                                 from order in g
                                 group order by new { GrpId = order.Element("OrderDate").Value } into g2
                                 select new XElement("Order",new XAttribute("ID",g2.Key.GrpId))
            );

<Sales>
<Sale>
<Employee ID="279">
<Order ID="12345">
<Product ID="776"/>
</Order>
<Order ID="1277">
<Product ID="776"/>
<Product ID="778"/>
</Order>                           
</Employee>
<Sale>
<Sales>

group EmployeeSalesDetail by new {  }