Set Value in TextBox On DropDownList Change depending on SelectedValue

Write a following code in page load event:-

 if (!IsPostBack)         {             SqlConnection cnn = new SqlConnection("Write SQL connection string");             SqlCommand cmd = new SqlCommand("write SQL command ", cnn);             SqlDataAdapter da = new SqlDataAdapter(cmd);             DataSet ds = new DataSet();             da.Fill(ds);             DropDownList1.AppendDataBoundItems = true;             DropDownList1.Items.Add(new ListItem { Text = "---please select--", Value ="0" });             DropDownList1.DataSource = ds;             DropDownList1.DataTextField = "appt database coloum name";             DropDownList1.DataBind();

        }