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)