create table t (pk number primary key,name varchar2(20));

create sequence t_seq ;
create trigger t_trigger before insert on t for each row
begin
select seq.nextval into :new.pk from dual;
 

end;
/

测试：select * from t;
        未选定行

       
SQL> insert into t(name) values ('aa');

已创建 1 行。

SQL> select * from t;

        PK NAME
---------- ----------------------------------------
         1 aa

SQL> insert into t(name) values ('bb');

已创建 1 行。

SQL> select * from t;

        PK NAME
---------- ----------------------------------------
         1 aa
         2 bb