LeetCode SQL: Second Highest Salary

select if(count(salary) = 0, NULL, salary) as `salary` from (
select salary from Employee  group by salary order by salary desc limit 1,1) tmp

Write a SQL query to get the second highest salary from the Employee table.

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

For example, given the above Employee table, the second highest salary is 200. If there is no second highest salary, then the query should return null.

对于null的处理稍烦