DropDownList to activate SQL Query in code hehind

I have a DDL that has 5 options in it. Based on the options a different query is run and bound to
a detailsview. At this point the DDl does not fire the query.
Here's the DDl from the page and the code behind


<asp:DropDownList ID="ddlQuantity" runat="server"
                    onselectedindexchanged="ddlQuantity_SelectedIndexChanged">
                    <asp:ListItem Value="1">Quantity 1</asp:ListItem>
                    <asp:ListItem Value="2">Quantity 2</asp:ListItem>
                &n ...