Two text boxes for search parmaters using one box or the other...

I'm working on a form with two text boxes for search parameters against a SQL table.  I've been able to
accomplish the objective of retrieving data from the database as long
as values are entered into to both text boxes and the query
string parameter is formed as such.  Where I'm falling down is when a
value is entered into one or the other text boxes; the result is that that no rows are being returned to the grid at all.   Hopefully this
will give you enough to help me troubleshoot this and point me the right direction.

<asp:TextBox ID="codetextbox" runat="server" OnTextChanged="codetextbox_TextChanged" </asp:TextBox>    
<asp:TextBox ID="desctextbox" runat="server"  OnTextChanged="desctextbox_TextChanged"</asp:TextBox>
<asp:Button ID="searchbutton" runat="server" OnClick="searchbutton_Click&quo ...