一个关于SQL外键约束的问题

            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["cat"].ConnectionString);

            con.Open();



            DataSet dataset = new DataSet();



            SqlDataAdapter ac = new SqlDataAdapter("select * from child",con);

            SqlCommandBuilder bc = new SqlCommandBuilder(ac);

            ac.Fill(dataset, "child");  //子表绑定到DataSet中



            SqlDataAdapter ap = new SqlDataAdapter("select * from Parent", con);

            SqlCommandBuilder bp = new SqlCommandBuilder(ap);

            ap.Fill(dataset, "Parent");//父表绑定到DataSet中



            DataColumn parent = dataset.Tables["Parent"].Columns["ID"];

            DataColumn child = dataset.Tables["child"].Columns["ID"];

            ForeignKeyConstraint fk = new ForeignKeyConstraint("f", parent, child);

            fk.UpdateRule = Rule.Cascade;

            dataset.Tables["child"].Constraints.Add(fk);//建立约束关系



            dataset.Tables["Parent"].Rows[0]["ID"] = 3;//改变父表中ID



            ap.Update(dataset, "Parent");//更新父表

            ac.Update(dataset, "child");//更新子表



            foreach (DataRow d in dataset.Tables["child"].Rows)

            {

                Console.WriteLine("{0}|{1}", d["ID"].ToString(), d["name"].ToString());

            }//显示DataSet中子表的值

            con.Close();