create table musteri(
id number not null,
adSoyad varchar2(30),
);

create sequence musteri_id_seq start with 1 increment by 1;

create or replace trigger musteri_insert
before insert on musteri
for each row
begin
select musteri_id_seq.nextval into :new.id from dual;
end;

insert into musteri (adSoyad) values ('mehmet zeki');
1 row created.
insert into musteri (adSoyad) values ('Mustafa günaltay');
1 row created.