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”