>> I have 2 files containing Id numbers and surnames (these files
essentially contain the same data) <<

Since these are files and not tables, as you just said, why not use a
file difference utility? Now if you mean that you have tables, then
we can give you a query. Please post DDL, so that people do not have
to guess what the keys, constraints, Declarative Referential
Integrity, datatypes, etc. in your schema are. Sample data is also a
good idea, along with clear specifications -- what does "partial
match" mean?? In Full SQL-92, that is a reserved word with a definite
meaning.