CREATE TABLE [dbo].[AltKategori](
	[id] [int] NULL,
	[KategoriId] [int] NULL,
	[AltKategori] [varchar](50) NULL
) 


CREATE TABLE [dbo].[Kategori](
	[id] [int] NULL,
	[Kategori] [varchar](50) NULL
)

    <asp:DropDownList ID="DropDownListKategori" runat="server" AutoPostBack="True" OnSelectedIndexChanged="DropDownListKategori_SelectedIndexChanged">
    </asp:DropDownList>
    <br />
    <asp:DropDownList ID="DropDownListAltKategori" runat="server">
    </asp:DropDownList>

protected void Page_Load(object sender, EventArgs e)
    {
        try
        {
            if (!Page.IsPostBack)
            {
                string connectionString = ConfigurationManager.ConnectionStrings["VeriConnectionString"].ConnectionString;
                SqlConnection conn = new SqlConnection(connectionString);
                conn.Open();

                SqlCommand cmd = new SqlCommand("SELECT id, Kategori FROM Kategori", conn);
                cmd.CommandType = CommandType.Text;

                SqlDataAdapter da = new SqlDataAdapter(cmd);
                DataTable dt = new DataTable();
                da.Fill(dt);

                DropDownListKategori.DataTextField = "Kategori";
                DropDownListKategori.DataValueField = "id";

                DropDownListKategori.DataSource = dt;
                DropDownListKategori.DataBind();
                DropDownListKategori.Items.Insert(0, new ListItem("– Seçiniz –", "0"));

                conn.Close();
            }

        }
        catch (Exception ex)
        {
            //Eğer log katmanınız varsa yazabilirsiniz oraya
        }
    }

    protected void DropDownListKategori_SelectedIndexChanged(object sender, EventArgs e)
    {
        try
        {
            string connectionString = ConfigurationManager.ConnectionStrings["VeriConnectionString"].ConnectionString;
            SqlConnection conn = new SqlConnection(connectionString);
            conn.Open();
            SqlCommand cmd = new SqlCommand("SELECT id, AltKategori FROM AltKategori WHERE KategoriId = @KategoriId", conn);
            cmd.CommandType = CommandType.Text;
            cmd.Parameters.Add(
                new SqlParameter("@KategoriId", Convert.ToInt32(DropDownListKategori.SelectedValue))
                );

            SqlDataAdapter da = new SqlDataAdapter(cmd);
            DataTable dt = new DataTable();
            da.Fill(dt);

            DropDownListAltKategori.DataTextField = "AltKategori";
            DropDownListAltKategori.DataValueField = "id";

            DropDownListAltKategori.DataSource = dt;
            DropDownListAltKategori.DataBind();
           // DropDownListAltKategori.Items.Insert(0, new ListItem("– Seçiniz –", "0"));

            conn.Close();

        }
        catch (Exception exp)
        {
            //Eğer log katmanınız varsa yazabilirsiniz oraya
        }
    }