DOM--sql server

   public List<LianHeData> select(int ID)
        {
            List<LianHeData> list = new List<LianHeData>();
            conn.Open();
            cmd = conn.CreateCommand();
                //*************************************************************************************************
            cmd.CommandText = "select a.Name name1,b.Name name2,c.Name name3 from Users a,Nation b,Clas c where a.ClassCode=b.Code and a.NationCode=c.Code and a.ID=@ID";
                          //**********************************************************************************************
            cmd.Parameters.Clear();            
            //放入到缓存中
            cmd.Parameters.Add("@ID", ID);
            SqlDataReader dr = cmd.ExecuteReader();

            while (dr.Read())
            {
                LianHeData data = new LianHeData();
                data.Name11 = dr["name1"].ToString();
                data.Name21 = dr["name2"].ToString();
                data.Name31 = dr["name3"].ToString();
                //data.ID1 = int.Parse(dr["ID"].ToString());
                //data.Name1 = dr["Name"].ToString();
                //data.PWD1 = dr["PWD"].ToString();
                //data.QuanXian1 = int.Parse(dr["QuanXian"].ToString());
                //data.NationCode1 = int.Parse(dr["NationCode"].ToString());
                //data.ClassCode1 = int.Parse(dr["ClassCode"].ToString());
                list.Add(data);
              
            }
            return list;
           
        }

"select a.Name,b.Name,c.Name from Users a,Nation b,Clas c where a.ClassCode=b.Code and a.NationCode=c.Code and a.ID=@ID"是查不出数据的  

必须定一个定量“select a.Name name1,b.Name name2,c.Name name3 from Users a,Nation b,Clas c where a.ClassCode=b.Code and a.NationCode=c.Code and a.ID=@ID”