改编自MSDN，如何使用EF 4退出联接

var query = from u in usergroups
            join p in UsergroupPrices on u.UsergroupID equals p.UsergroupID into gj
            from x in gj.DefaultIfEmpty()
            select new { 
                UsergroupID = u.UsergroupID,
                UsergroupName = u.UsergroupName,
                Price = (x == null ? String.Empty : x.Price) 
            };