Scalar function: format address

Hey everybody,

I'm beging in DB2 and I need creat some scalar function but I don't know how to start here is my exercise:

Define a scalar function, which takes the ID of an address as input parameter and return a string in the format "user@domain", e.g. "john@foobar.com".

Function name: format_address
Input: addressId of type "ref(address_t)"
Output: formatted address string of type "varchar"

Someone knows how can I to do that?