SQL joining question

I need to be able to select records from 2 identical files that have a
1-to-1 relationship. However, there are some records in file1 that isn't in
file2 and vise-versa... and I want the entire set. I need to do this as a
dynamic SQL statement in a RPG program. As an example:

file1
custno  inv     amountdue
1111            123             200
5555            120             200


file2
custno  inv     amountpaid
1111            123             050
6666            125           500

the result cursor would be:
custno  inv     amountdue       amountpaid
1111            123             200              050
5555            120             200              000
6666            125             000              500

What kind of join can be done for this? I have been unsuccessful so far in
figuring it out.

Any help would be appreciated.

Mike