SQL> create table scott.t1 (id number, val varchar2(8));
Table created.
SQL> CREATE SEQUENCE scott.t1_id_seq INCREMENT BY 1 START WITH 1 NOMAXVALUE NOCYCLE ;
Sequence created.
SQL> create or replace trigger scott.t1_id_trigger
2 before insert on scott.t1 for each row
3 begin
4 select scott.t1_id_seq.nextval into :new.id from dual;
5 end;
6 /
Trigger created.
SQL> insert into scott.t1(val) values('a');
1 row created.
SQL> commit;
Commit complete.
SQL> select * from scott.t1;
ID VAL
---------- ------------------------
1 a
SQL> insert into scott.t1(val) values('b');
1 row created.
SQL> select * from scott.t1;
ID VAL
---------- ------------------------
1 a
2 b
SQL> commit;
Commit complete.