Add Items 2 dropdownlist on selection of other dropdowllist

if (IsPostBack == false)
                 {

                     SqlCommand com = new SqlCommand("your query", con object);
                     SqlDataReader ddlValues;
                     con.Open();
                     ddlValues = com.ExecuteReader();

                     DropDownList1.DataSource = ddlValues;
                     DropDownList1.DataValueField = ".......";
                     DropDownList1.DataTextField = ".......";
                     DropDownList1.DataBind();
                     con.Close();
                 }

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    { SqlCommand com = new SqlCommand("query", con);
        
        SqlDataReader ddlValues;
        con.Open();
        ddlValues = com.ExecuteReader();

        DropDownList2.DataSource = ddlValues;
        DropDownList2.DataValueField = "column";
        DropDownList2.DataTextField = "column";
        DropDownList2.DataBind();
        con.Close();
}