update listbox multiple selected items in database

hi ,
i am working on silverlight 4 using mvvm.i am using listbox in my application.name of a listbox is role type.in this i have set the selection mode multiple.and i have one textbox also by employee name.
and in my database i am having a table by name role,in the table i am having 2 fields by name employee name and roletype.
now in my application i am able to do insert,read and delete.
but in update i am getting poblem.
i need whenever i select a record from datagrid, then in my listbox shows the no. of role type items selected of that employee and when i change a multiple selected items in listbox of that employee,then on update button click it saves updated data in database and dont which was'nt selected on edit mode.
plz give solution as soon as possible.