PL/SQL :: Getting Rate For Report?

This is my sample code:

create table myReport(
report_id integer,
product_class_id integer,
place_id integer,

[code]...

would like to get the rate for my report table base. The search for the rate first will be base on product_class_id, year and place_id, second if the report row match the product_class_id and year but not place_id it should get the default rate (in my rate table the place_id is null).

Last if the report row doesn't match any key the result should be zero.This is my result query, how can I do this query?
report_id     product_class_id     place_id     fy rate
1     1     1     2012   15
2     1     2     2011   6
3     1     3     2011   7
4     2     2     2012   18
5     2     5     2011   2
6     3     1     2012   0
7     4     1     2012   0

I try to do a function by don't know how to handle the default place_id (null)