UPSERT without Primary Key

Is it possible to do an UPSERT with a subquery without referencing the Primary Key? From the documentation it seems like this would just lead to an insert:

"The UPSERT or REPLACE statement with a subquery works like the INSERT statement, except that if an old row in the table has the same value as a new row for a PRIMARY KEY, then the old row is changed by values of the returned record from a subquery. Unless the table has a PRIMARY KEY, it becomes equivalent to INSERT because there is no index to be used to determine whether or not a new row duplicates another."

Is there an equivalent for this?

create column table table1 (key1 varchar2(10) primary key, field1 varchar2(10), field2 varchar2(10));
create column table table2 (key1 varchar2(10) primary key, field1 varchar2(10), field2 varchar2(10));
insert into table1 values ('1', 'test1', 'test1');
insert into table2 values ('2', 'test1', 'test2');
merge into table1 t1 using (select key1, field1, field2 from table2) t2 on (t1.field1 = t2.field1)
when matched then
update set t1.field2 = t2.field2 
when not matched then insert (t1.key1, t1.field1, t1.field2) values (t2.key1, t2.field1, t2.field2);

So field2 for the single row in table1 would be updated based on the join to field1 rather than the primary key.

Thanks,

Jason