直接查詢如果不存在第二高的Salary不會返回值，所以應該用子查詢

select (select distinct Salary from Employee order by Salary desc limit 1,1) as SecondHighestSalary

也可以用IFNULL
ifnull(exr1,exr2) 如果exr1是null那麼返回exr2否則返回exr1

select ifnull( (select distinct Salary from Employee order by Salary desc limit 1 offset 1), NULL) as SecondHighestSalary
 

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