xml save and update

[New Learner]

Hi there!
I wish that someone helps me in the next code :
A- Before adding a new data, I want to check if the (elev_no) or the (id) exists before , if it already exists in the xml database then message (the student or the id exists) , if not then add the new data.
B- Before updating the existed data, if the user tries to change or modify the (elev_no) or the (id) which is already exist in the xml database then message (the student or the id exists) , if not then update the data.

[New Learner]

And now here it is my code :
strName = Input.GetText("name");
strNo = Input.GetText("Elev_No");
strPhone = Input.GetText("phone");
strAdresse = Input.GetText("el_adresse");

--XML.InsertXML("database/eleve", "<eleve>\n<name>"..strName.."</name>\n<elev_no>"..strNO.."</elev_no>\n</eleve>\n", XML.INSERT_BEFORE);

strXML = "<eleve id='"..strNo.."'>\n<name>"..strName.."</name>\n<elev_no>"..strNO.."</elev_no>\n";
strXML = strXML.."<phone>"..strPhone.."</phone>\n<adresse>"..strAdresse.."</adresse>\n</eleve>\n";

-- adding a new student

if add_or_edit == "add" then

nCount = XML.Count("database", "eleve");

XML.InsertXML("database/eleve:"..nCount, strXML, XML.INSERT_AFTER);

-- modifing a student

elseif add_or_edit == "edit" then
local tbSelect = ListBox.GetSelected("Listcontent");
XML.InsertXML("database/eleve:"..tbSelect[1], strXML, XML.REPLACE);
end
--XML.Save("AutoPlay\\Docs\\elev_data.xml");

XML.Save(_SourceFolder.."\\AutoPlay\\Docs\\elev_da ta.xml");

ListBox.DeleteItem("Listcontent", LB_ALLITEMS);
fill_list()
Button.SetEnabled("Add", true);
Button.SetEnabled("Edit", true);
Button.SetEnabled("save", false);