How to display data dynamically based on previous selected data

Hi,

I have two dropdowns in my interface. Based on first selected dropdown second dropdown has to chaange.

like, First dropdown is 'Country' . If i select the country it should display list of states for that country.

I have created multiple constants for states for each and every country.

My constants are 

APPNAME_COUNTRY - List of countries

APPNAME_COUNTRYNAME1 - list of states for that country

APPNAME_COUNTRYNAME2 - List of states for countryname2

So if i select country i have to populate another dropdown (states) in Interface. So how can i call the constants dynamically. Below is the validation i tried but no luck. Please help.

if (
isnull(ri!APPNAME_Customer_Details.address.country),
null,
cons!BP_+upper(ri!APPNAME_Customer_Details.address.country)
)

Regards,

Sirisha

Regards,

Sirisha