题目：获查找当前薪水(to_date=‘9999-01-01’)排名第二多的员工编号emp_no、薪水salary、last_name以及first_name，不准使用order by
解题关键：要求的是薪水排名第二多的，所以关键是找到第二多的薪水对应的salary值
1.可以通过MAX配合<运算得到
2.可以通过自连接和<=号配合COUNT函数找出排名高的薪水
补充：第二种方法可以推广到求薪水排名第N高的情况

方法1：用MAX函数，先查出最大salary，再利用<得到不含最大salary的子表，在子表上再求最大值
SELECT e.emp_no, s.salary, e.last_name, e.first_name
FROM employees AS e
INNER JOIN salaries AS s
ON e.emp_no=s.emp_no
WHERE to_date='9999-01-01'
AND salary=(SELECT max(salary)
FROM salaries
WHERE to_date='9999-01-01'
AND salary<(SELECT MAX(salary)
FROM salaries
WHERE to_date='9999-01-01')
);

方法2：通过自连接和<=号配合COUNT函数找出排名高的薪水
SELECT e.emp_no, s.salary, e.last_name, e.first_name
FROM employees AS e
INNER JOIN salaries AS s
ON e.emp_no=s.emp_no
WHERE to_date='9999-01-01'
AND salary=(SELECT s1.salary
FROM salaries AS s1
INNER JOIN salaries AS s2
ON s1.to_date='9999-01-01'
AND s2.to_date='9999-01-01'
WHERE s1.salary <= s2.salary
GROUP BY s1.salary
HAVING COUNT(DISTINCT s2.salary)=2
);