通过下拉列表选择

Question

如何从下拉列表中执行SELECT查询。我有一个包含文凭学位的下拉菜单，在我的数据库中，我有一个名为校友（Aid，...，文凭）的表和一个名为Diploma的表，其中包含（Did，Diploma），我们手动插入了5个文凭（brevet， bacc2，本科，硕士，博士）。我想要的是从下拉列表中选择一个文凭，以便在我确认或提交后可以在校友表中结束。这是我的代码。

Form.aspx：

<form id="form" runat="server" method="post">
<div>
    <table class="style1">
        <tr>
            <td>Diploma:</td>
            <td>
                <asp:DropDownList ID="Diploma" runat="server" AppendDataBoundItems="true" Width="160px">
                <asp:ListItem Text="Select Degree" Value="Select Degree"></asp:ListItem>
                </asp:DropDownList>
            </td>
        </tr>
      </table>      
<center>
<asp:Button ID="Button1" runat="server" Text="Next" OnClick="Button1_Click" Width="85px" />
</center>
</form>

Form.aspx.cs：

protected void Button_Click(object sender, EventArgs e)
{
    Session["diploma"] = Diploma.SelectedValue;

    if (Status.Text == "1")
    {
        Response.Redirect("C2.aspx");
    }
    else 
    {
        Response.Redirect("C1.aspx");
    }
}
protected void Page_Load(object sender, EventArgs e)
{
    if (!Page.IsPostBack)
    {
        if (Session["diploma"] != null &&)
        {
            Diploma.SelectedValue = Session["diploma"].ToString();
        }
    }
}

C1.aspx.cs：

    if (!Page.IsPostBack)
    {
        if (Session["pname"] != null && Session["plastname"] != null && Session["pos"] != null &&
        Session["children"] != null && Session["schoolofchildren"] != null &&
        Session["schoolofchildren1"] != null && Session["schoolofchildren2"] != null)
        {
            PName.Text = Session["pname"].ToString();
            PLastName.Text = Session["plastname"].ToString();
            OldStudent.Text = Session["pos"].ToString();
            Children.Text = Session["children"].ToString();
            SchoolOfChildren.Text = Session["schoolofchildren"].ToString();
            SchoolOfChildren1.Text = Session["schoolofchildren1"].ToString();
            SchoolOfChildren2.Text = Session["schoolofchildren2"].ToString();
        }
    }
}
protected void Button2_Click(object sender, EventArgs e)
{
    Session["pname"] = PName.Text;
    Session["plastname"] = PLastName.Text;
    Session["pos"] = OldStudent.Text;
    Session["children"] = Children.Text;
    Session["schoolofchildren"] = SchoolOfChildren.Text;
    Session["schoolofchildren1"] = SchoolOfChildren1.Text;
    Session["schoolofchildren2"] = SchoolOfChildren2.Text;

    Response.Redirect("Form.aspx");
}
protected void Button3_Click(object sender, EventArgs e)
{
    Session["pname"] = PName.Text;
    Session["plastname"] = PLastName.Text;
    Session["pos"] = OldStudent.Text;
    Session["children"] = Children.Text;
    Session["schoolofchildren"] = SchoolOfChildren.Text;
    Session["schoolofchildren1"] = SchoolOfChildren1.Text;
    Session["schoolofchildren2"] = SchoolOfChildren2.Text;

    Response.Redirect("C2.aspx");
}
protected void Button4_Click(object sender, EventArgs e)
{
    try
    {
        ExecuteInsert1(Session["diploma"].ToString());

        ExecuteInsert2(PName.Text, PLastName.Text, OldStudent.SelectedValue, Children.SelectedValue, SchoolOfChildren.Text, SchoolOfChildren1.Text,
        SchoolOfChildren2.Text);
        Response.Write("Successfully Added!");
        ClearControls(Page);
        Session.Clear();
    }
    catch{}
}
public string GetConnectionString()
{
    return System.Configuration.ConfigurationManager.ConnectionStrings["Champville1ConnectionString"].ConnectionString;
}
public static void ClearControls(Control Parent)
{
    if (Parent is TextBox)
    { (Parent as TextBox).Text = string.Empty; }
    else
    {
        foreach (Control c in Parent.Controls) ClearControls(c);
    }
}
private void ExecuteInsert1(string diploma)
{
    SqlConnection conn = new SqlConnection(GetConnectionString());
    string sql = "INSERT INTO Alumni (Diploma) VALUES " + " (@Diploma)";

    try
    { 
        conn.Open();
        SqlCommand cmd = new SqlCommand(sql, conn);
        SqlParameter[] param = new SqlParameter[1];

        param[0] = new SqlParameter("@Diploma", SqlDbType.NVarChar, 50);

        param[0].Value = diploma;

        for (int i = 0; i < param.Length; i++)
        {
            cmd.Parameters.Add(param[i]);
        }
        cmd.CommandType = CommandType.Text;
        cmd.ExecuteNonQuery();
    }
    catch (System.Data.SqlClient.SqlException ex)
    {
        string msg = "Insert Error:";
        msg += ex.Message;
        throw new Exception(msg);
    }
    finally
    {
        conn.Close();
    }
}
private void ExecuteInsert2(...){}