編寫一個 SQL 查詢，獲取 Employee 表中第二高的薪水（Salary） 。
+----+--------+
| Id | Salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+
例如上述 Employee 表，SQL查詢應該返回 200 作為第二高的薪水。如果不存在第二高的薪水，那么查詢應返回 null。
+---------------------+
| SecondHighestSalary |
+---------------------+
| 200 |
+---------------------+
來源：力扣（LeetCode）
select max(Salary) SecondHighestSalary
from employee
where
salary<(select max(salary) from employee)
select (select distinct salary from Employee order by salary desc limit 1,1) as SecondHighestSalary