​​Employee​​

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

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

题目答案：

# Write your MySQL query statement below
SELECT max(Salary) FROM Employee WHERE Salary < (SELECT max(Salary) FROM Employee)

题目解析：

SELECT max(Salary) FROM Employee WHERE Salary < (SELECT max(Salary) FROM Employee)

Using max() will return a NULL if the value doesn't exist. So there is no need to UNION a NULL. Of course, if the second highest value is guaranteed to exist, using LIMIT 1,1 will be the best answer.