adding unique identifier in combobox


I have a database table name country .

id name
10 norway
25 japan
36 india
49 australia

i have to set the data of table into combobox so that my combobox should display country name {norway,japan,india,australia}.i already done if i have to set only value.
But if i have to set hidden field say id then how can i set that id. if i select japan from combobox it should return its id value to 25

pls help.
thanks in advance.