写SQL查询语句获得Employee表中第二高的薪水
+----+--------+ | Id | Salary | +----+--------+ | 1 | 100 | | 2 | 200 | | 3 | 300 | +----+--------+
例如，给定如上Employee表，查询语句应该返回200作为它的第二高薪水。如果不存在第二高薪水就返回null
+---------------------+ | SecondHighestSalary | +---------------------+ | 200 | +---------------------+
1：利用MAX()的简单子查询
SELECT MAX(Salary) AS SecondHighestSalary FROM Employee WHERE Salary<(SELECT MAX(Salary) FROM Employee)2：利用ORDER BY ... DESC和LIMIT 1 OFFSET 1进行查询（参考：https://blog.csdn.net/yeruby/article/details/47748165）
select(SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT 1 OFFSET 1) AS SecondHighestSalary算法题来自：https://leetcode-cn.com/problems/second-highest-salary/description/