Second Highest Salary（选择第二高的工资）

要求：

For example, given the above Employee table, the query should return 

200

 as
the second highest salary. If there is no second highest salary, then the query should return 

null

.

Create table If Not Exists Employee (Id int, Salary int);
Truncate table Employee;
insert into Employee (Id, Salary) values ('1', '100');
insert into Employee (Id, Salary) values ('2', '200');
insert into Employee (Id, Salary) values ('3', '300');

方法一：
select IF(
(select count(DISTINCT Salary) from Employee)>1
,(select Salary from Employee order by Salary desc limit 1,1)
,null)
as SecondHighestSalary;

方法二：
SELECT max(Salary)
FROM Employee
WHERE Salary < (SELECT max(Salary) FROM Employee)