题目2：MySQL----------Second Highest Salary

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.

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.