2 Options:
1. Use Dropdown selectedIndexchange event (with AutoPostback set to true) on server side to find the value selected in the dropdownlist. based on the value selected, get the data from database and display in a grid.
2. Use AJAX to fire a XMLHttpRequest/Callback/PageMethod from client side to server on drodown change and get the data back as a response which can be used to bind with the XML.
Try both and learn on.
Option 1 is straightforward. Option 2 needs knowledge on ways to interact with server form client, read here:
1.
XMLHttpRequest[
^]
2.
Callback[
^]
3.
WebService call[
^]
4.
PageMethod[
^]