LINQ中的左外部联接

List<T> left = GetLeft();
List<T> right = GetRight();

IEnumerable result = // Have no idea

var result = left.Except(right);

   Select c.Name from Customers c             
   Left Outer Join Purchases p on c.customerid=p.customerid 
   where p.price is null

   from c in Customers
   join p in Purchases on c.customerid=p.customerid into custPurchases
   from cp in custPurchases.DefaultIfEmpty()
   where cp==null
   select new
   {
   cc.Name
   }