//Functions  left join Resos
                var result = (from x in Resos
                             join y in RoleFunctionMappings on x.Id equals y.FunctionId
                             into z
                             from y in z.DefaultIfEmpty()
                             select new
                             {
                                 ResourceValue = x.ResourceValue,
                                 ParentFunctionId = x.ParentFunctionId,
                                 x.PagePath,
                                 Id = x.Id,
                                 x.FunctionType,
                                 FunctionCode = x.FunctionCode,
                                 MappingId = y == null ? -1 : y.Id,
                                 IsDeny = y == null ? false : y.IsDeny,
                                 IsDefaultFunction = y == null ? false : y.IsDefaultFunction,
                                  x.Level ,
                                 x.ShowSequence
                             }).OrderBy(Item=>Item.Level).ThenBy(item=>item.ShowSequence);

------解决方案--------------------
C# code

var query= from to in db.hotel_user_order
           join t1 in db.hotelinfo
           on to.hotelid equals t1.id into left
           from t1 in left.DefaultIfEmpty()
           group to by new {to.hotelid,t1.cname} into g
           select new 
           { 
              hotelid=g.Key.hotelid,
              sellCount=g.Count(),
              cname=g.Key.cname
           };

------解决方案--------------------
探讨


C# code

var query= from to in db.hotel_user_order
           join t1 in db.hotelinfo
           on to.hotelid equals t1.id into left
           from t1 in left.DefaultIfEmpty()
           group to ……