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.
