思想：

題目要求查詢當前薪水(to_date='9999-01-01')排名第二多的員工編號emp_no、薪水salary、last_name以及first_name，不準使用order by。首先通過條件e.emp_no=s.emp_no將兩表內連線相連，然後通過條件to_date='9999-01-01'取出員工當前薪水，再者通過子查詢找出當前最高薪水select max (salary) from salaries where to_date='9999-01-01'，最後通過條件salary not in
(select max (salary) from salaries where to_date='9999-01-01')且max(salary)找出第二高薪水。

select e.emp_no, max(s.salary) as salary, e.last_name, e.first_name
from employees e inner join salaries s on e.emp_no=s.emp_no
where to_date='9999-01-01'and salary not in
(select max (salary) from salaries where to_date='9999-01-01');