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;