数据库查询-编写一个 SQL 查询，获取 Employee 表中第二高的薪水（Salary）

解题思路：

找出第二高的薪水，先找出表里的最大值e1.Salary，再找出表里的最大值e2.Salary，两者进行比较，e1.Salary<e2.Salary，找出的e1.Salary即是第二高的薪水

select max(Salary) as SecondHighestSalary from Employee e1 where 
(select max(Salary) from Employee e2 where e1.Salary<e2.Salary)

另外一种简洁的方法

select max(Salary) as SecondHighestSalary from Employee 
where Salary<(select max(Salary) from Employee)

第三种方法

select IFNULL((select Distinct Salary from Employee order by Salary DESC limit 1,1),null) 
as SecondHighestSalary