public static void UpdateLoaiDocGia(int idLoaiDG, string tenLoaiDG)
        {
            LOAIDOCGIA loaiDG = db.LOAIDOCGIAs.Find(idLoaiDG);

            loaiDG.TenLoaiDocGia = tenLoaiDG;
            db.SaveChanges();
        }
        public static void DeleteLoaiDocGia(int idLoaiDG)
        {
            LOAIDOCGIA loaiDG = db.LOAIDOCGIAs.Find(idLoaiDG);

            db.LOAIDOCGIAs.Remove(loaiDG);
            db.SaveChanges();
        }
        public static void AddLoaiDocGia(string tenLoaiDG)
        {
            LOAIDOCGIA loaiDG = new LOAIDOCGIA();

            loaiDG.TenLoaiDocGia = tenLoaiDG;
            db.LOAIDOCGIAs.Add(loaiDG);
            db.SaveChanges();
        }