CREATE SEQUENCE seq_test MINVALUE 1 INCREMENT BY 1 START WITH 1 NOCACHE ORDER NOCYCLE;

CREATE TABLE tab_test
(
id NUMBER(*,0),
nr NUMBER(*,0),
text VARCHAR2(25 CHAR)
);


merge into tab_test z
using (
       select 1 nr,
              'A' text
         from dual
      ) q on (q.nr = z.nr)
when matched then update set
z.text = q.text
when not matched then insert
(id, nr, text)
values
(seq_test.nextval, q.nr, q.text)
;

select * from tab_test;
-- => 1 | 1 | A
select seq_test.currval from dual;
-- => 1

merge into tab_test z
using (
       select 1 nr,
              'B' text
         from dual
      ) q on (q.nr = z.nr)
when matched then update set
z.text = q.text
when not matched then insert
(id, nr, text)
values
(seq_test.nextval, q.nr, q.text)
;

select * from tab_test;
-- => 1 | 1 | B
select seq_test.currval from dual;
-- => 2  value of sequence did change without inserting data

merge into tab_test z
using (
       select 2 nr,
              'C' text
         from dual
      ) q on (q.nr = z.nr)
when matched then update set
z.text = q.text
when not matched then insert
(id, nr, text)
values
(seq_test.nextval, q.nr, q.text)
;

select * from tab_test;
-- => 1 | 1 | B
-- => 3 | 2 | C
select seq_test.currval from dual;
-- => 3

CREATE OR REPLACE FUNCTION mseq RETURN INTEGER 
RESULT_CACHE
IS
BEGIN
  RETURN seq_test.nextval;
END;