Data Access Layer

http://blog.webwizo.com/2010/06/05/creating-data-access-layer-aspnet-csharp-part-1/

http://www.dotnetbips.com/articles/fbea3517-b0c8-4e79-bc52-cc75e0b1b86f.aspx

ASP .NET

http://codeasp.net/articles/cascading-dropdownlist-using-jquery-and-asp-net/214/cascading-dropdownlist-using-jquery-and-asp-net

http://www.dotnetfunda.com/articles/article71.aspx

http://www.aspsnippets.com/Articles/AJAX-Cascading-DropDownList-using-jQuery-in-ASP.Net.aspx

http://sushantp.wordpress.com/2009/02/22/aspnet-fill-and-show-dependent-drop-down-list-on-client-side-without-server-control-and-postback/

http://www.asp.net/data-access/tutorials