I believe you can use the following (I am not on a machine with VFP so syntax may be a little off but this is the technique)

assumes following:
1. both a and b have same or similar fields if only similar then would have to request fields rather than use *

2. there is a key value common to both a and b call it x

then can create following sql:

select * from a ;
union ;
select * from b where not x in (select distinct x from a)

Please check above against the valid VFP syntax but that will retrieve all fields in a and all fields in b where x is in b but not in a.