create table member(
memberId number primary key,
memberMail varchar2(20)not null,
memberName varchar2(20) not null,
memberPassword varchar2(20)
);

CREATE SEQUENCE emp_sequence
INCREMENT BY 1 -- 每次加幾個
START WITH 1 -- 從1開始計數
NOMAXVALUE -- 不設定最大值
NOCYCLE -- 一直累加，不迴圈
NOCACHE -- 不建緩衝區

create trigger mem_trig before
insert on member for each row when (new.memberId is null)
begin 
 select emp_sequence.nextval into:new.memberId from dual; 
 end;