第二高的薪水

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

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

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