我定义了一个datagridview（没有绑定）和一个combobox，自己在datagridview中构造了一个表（两列），现在从combobox中选择一项，第一列显示combobox的选项，第二列显示从数据库中匹配的值，请问应该如何给第二列赋值呢？
private DataSet CreateDataSet()
{
string sqlstr = "Select phone from student where studentid='" + comboBox1.Text + "'";
SqlConnection cn = new SqlConnection("server=(local);integrated security=true;database=studentmanagement;");
cn.Open();
SqlDataAdapter da = new SqlDataAdapter(sqlstr, cn);

DataSet ds = new DataSet();
da.Fill(ds, student);
DataTable dt = new DataTable("student");

DataColumn dc = dt.Columns.Add("id",typeof(string));



dt.Columns.Add("phone", typeof(string));


DataRow dr = dt.NewRow();
dr["id"] = comboBox1.SelectedItem.ToString();
dr["phone"] = ; //这里应该怎么写？
dt.Rows.Add(dr);

ds.Tables.Add(dt);
return ds;
//cn.Close();
}






private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
{
this.dataGridView1.DataSource = CreateDataSet();
this.dataGridView1.DataMember = "student";


}