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 query should return 200 as the second highest salary. If there is no second highest salary, then the query should return null.

+---------------------+

| SecondHighestSalary |

+---------------------+

| 200                 |

+---------------------+

思路为先把最大的选出来, 然后把不等于之前的最大的, 最大的salary选出来即可.

SELECT max(Salary) as SecondHighestSalary FROM Employee WHERE Salary NOT IN (SELECT max(Salary) FROM Employee)