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');
 

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

SELECT max(Salary)
FROM Employee
WHERE Salary < (SELECT max(Salary) FROM Employee)