SqlConnection cn = new SqlConnection(cnStr);
            ds = new DataSet();

            string sqlStr = "SELECT yxdm, yxmc  FROM  jc_jgb order by yxdm";
           myDataAdapter = new SqlDataAdapter(sqlStr, cn);
            myDataAdapter.Fill(ds, "jc_jgb");

            sqlStr = "SELECT yxdm,jcbh,jcmc,zz,dj FROM jc_kcb order by yxdm,jcmc ";
            myDataAdapter = new SqlDataAdapter(sqlStr, cn);
             myDataAdapter.Fill(ds, "jc_kcb");


            sqlStr = "SELECT  yxdm,zydm,zymc,xz FROM jc_zyb order by yxdm,zydm ";
            myDataAdapter = new SqlDataAdapter(sqlStr, cn);
            myDataAdapter.Fill(ds, "jc_zyb");

 
 


            //DataColumn parentColumn = ds.Tables["jc_jgb"].Columns["yxdm"];
            //DataColumn childColumn = ds.Tables["jc_kcb"].Columns["yxdm"];
            //创建关系
            DataRelation rel_jg_jc = new DataRelation("jg_jc", ds.Tables["jc_jgb"].Columns["yxdm"], ds.Tables["jc_kcb"].Columns["yxdm"]);

            // 添加关系到dataset
            ds.Relations.Add(rel_jg_jc);

            DataRelation rel_jg_zy = new DataRelation("jg_zy", ds.Tables["jc_jgb"].Columns["yxdm"], ds.Tables["jc_zyb"].Columns["yxdm"]);

            // 添加关系到dataset
            ds.Relations.Add(rel_jg_zy);
            

            // 将数据集中的Customers表绑定到DataGrid控件上
            dataGridView2.DataSource = ds.Tables["jc_jgb"].DefaultView;
            dataGridView3.DataSource = ds.Tables["jc_jgb"].DefaultView;
            dataGridView3.DataMember = rel_jg_jc.ToString();
            dataGridView4.DataSource = ds.Tables["jc_jgb"].DefaultView;
            dataGridView4.DataMember = rel_jg_zy.ToString();



请问红色那两句中的绿色的字段是什么来的，怎样定义的？为什么改了它之后就运行出错，谢谢！