Forms Data Controls :: Binding SQL Query Results To A Gridview?

I have a gridview that I want to bind the results of a query to it once a user selects a value from a drop down list.

The query selects the same row from the same table but selects different fields based on the drop down list value selected

I get to the query selection but it does not refresh that page with the populated gridview.

Here's the code behind code

protected void ddlQuantity_SelectedIndexChanged(object sender, EventArgs e)
{
SqlConnection connection = null;
connection = new SqlConnection(ConfigurationManager.ConnectionStrings["PSI_DB_1ConnectionString"].ConnectionString.ToString());
connection.Open();
String nEstno = Request.QueryString["id"];
int id = Convert.ToInt32(Request.QueryString["id"]);
string SQLquery = "";
switch (int.Parse(ddlQuantity.Text))
{
case 1:
SQLquery = "Select Quan1"etc...;
break;
case 2:
SQLquery = "Select Quan2"etc...;
break;
case 3:
SQLquery = "Select Quan3"etc...;
break;
case 4:
SQLquery = "Select Quan4"etc...;
break;
case 5:
SQLquery = "Select Quan5"etc...;
break;
}
SqlCommand Cmd = new SqlCommand(SQLquery, connection);
SqlDataReader Dr = Cmd.ExecuteReader();
if (Dr.HasRows)
{
gvEstQty.DataBind();
}
Cmd.Dispose();
connection.Close();
connection.Dispose();
}