请问在面的代码是什么意思，有没有错误，我引用别人的代码的，但是报错

select floor(sysdate - to_date('20020405','yyyymmdd')) from dual;

create sequence users_id
minvalue 1
maxvalue 99999
start with 1
increment by 1;

create sequence admin_id
minvalue 1
maxvalue 99999
start with 1
increment by 1;

create sequence book_id
minvalue 1
maxvalue 99999
start with 1
increment by 1;

create sequence ub_id
minvalue 1
maxvalue 99999
start with 1
increment by 1;