create table tb_student
(
  id         NUMBER(10)      not null,
  createtime     DATE         not null,
  constraint PK_tb_student primary key (id)
);

comment on table "tb_student" is
'学生表';

comment on column "tb_student"."id" is
'主键id';

comment on column "tb_student"."createtime" is
'创建时间';


--创建序列
create sequence seq_tb_student
minvalue 1
nomaxvalue
start with 1
increment by 1
nocycle  --一直累加，不循环
nocache; 


--创建触发器，如果insert语句不指定ID自动插入增长值
CREATE OR REPLACE TRIGGER tr_tb_student 
BEFORE INSERT ON tb_student FOR EACH ROW WHEN (new.id is null)
begin
select seq_tb_student.nextval into:new.id from dual;
end;