merge into <table> t
using (
select * from <externaltable>
minus
select * from <table>
) diff
on (t.<pk_columns> = diff.<pk_columns>)
....

create table t_target (key1 number primary key, value1 varchar2(256));
insert into t_target values (1, 'DO NOTHING - same in source and target');
create table t_source as select * from t_target;
insert into t_source values (2, 'INSERT - in source, not in target');
insert into t_target values (3, '(update then) DELETE - not in source, in target');
insert into t_target values (4, 'DO NOTHING - update this from source');
insert into t_source values (4, 'UPDATE - put this in target');
select * from t_target order by 1
/
--KEY1                   VALUE1
------------------------ ------------------------------------------------
--1                      DO NOTHING - same in source and target
--3                      (update then) DELETE - not in source, in target
--4                      DO NOTHING - update this from source
--
--3 rows selected

select * from t_source order by 1
/
--KEY1                   VALUE1
------------------------ -------------------------------------
--1                      DO NOTHING - same in source and target
--2                      INSERT - in source, not in target
--4                      UPDATE - put this in target
--
--3 rows selected

merge into T_TARGET o
using (
  select a.*, sum(new_cnt - old_cnt) over(partition by KEY1) mod_type from (
    select KEY1, VALUE1, sum(old_cnt) old_cnt, sum(new_cnt) new_cnt from (
      select o.*, 1 old_cnt, 0 new_cnt from T_TARGET o
      union all
      select n.*, 0 old_cnt, 1 new_cnt from T_SOURCE n
    ) group by KEY1, VALUE1 having sum(old_cnt) <> sum(new_cnt)
  ) a
) n
on (o.KEY1 = n.KEY1)
when matched then update set VALUE1 = n.VALUE1
  where mod_type in (-1, old_cnt)
delete where mod_type = -1
when not matched then insert (KEY1, VALUE1) values (n.KEY1, n.VALUE1)
/
--3 rows merged
select * from t_target order by 1
/
--KEY1                   VALUE1
------------------------ -------------------------------------
--1                      DO NOTHING - same in source and target
--2                      INSERT - in source, not in target
--4                      UPDATE - put this in target
--
--3 rows selected