LeetCode176——第二高的薪水

+----+--------+
| Id | Salary |
+----+--------+
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |
+----+--------+

+---------------------+
| SecondHighestSalary |
+---------------------+
| 200                 |
+---------------------+

select Salary from Employee
  order by Salary desc
  limit 1, 1;

select Salary from Employee
  group by Salary
  order by Salary desc
  limit 1, 1;

select
  ifnull(
    (select Salary from Employee group by Salary order by Salary desc limit 1, 1),
    null
  ) as SecondHighestSalary;

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