关键字：level connect by level

一、实验一

--查询没有被使用过的工号。
with all_used_empno as
 (select level poss_empno
    from (select max(empno) max_empno from emp)
  connect by level <= max_empno)
select poss_empno
  from all_used_empno
 where poss_empno not in (select empno from emp)
 order by poss_empno;

实验二、得到1-10的序列值。

select level from dual connect by level <=10