编写一个 SQL 查询，获取 Employee 表中第二高的薪水（Salary） 。
+----+--------+ | Id | Salary | +----+--------+ | 1 | 100 | | 2 | 200 | | 3 | 300 | +----+--------+
例如上述 Employee 表，SQL查询应该返回 200 作为第二高的薪水。如果不存在第二高的薪水，那么查询应返回 null。
+---------------------+ | SecondHighestSalary | +---------------------+ | 200 | +---------------------+
答案1：使用临时表
SELECT (SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT 1 OFFSET 1) AS SecondHighestSalary ;
答案2：使用IFNULL
select ifnull((select distinct salary from employee order by salary desc limit 1,1),null) as SecondHighestSalary ;