一、問題描述：
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

二、問題求解

（1）方法一

#所謂的“第二高”是指去重後的僅次於最高值的分數
#因此只需藉助於MAX關鍵詞即可
select MAX(Salary)
from Employee
where Salary < (select MAX(Salary) from Employee);

（2）方法二

#distinct,order by 和limit 組合使用
select IFNULL(
    (select  distinct Salary as SecondHighSalary 
     from Employee order by Salary desc 
     limit 1
 
, 1),
    NULL
); 

三、DISTINCT語句去重複記錄

在MySQL查詢中，可能會包含重複值。這並不成問題，不過，有時您也許希望僅僅列出不同（distinct）的值。

關鍵詞 DISTINCT 用於返回唯一不同的值，就是去重。用法：

SELECT DISTINCT * FROM tableName

DISTINCT 這個關鍵字來過濾掉多餘的重複記錄只保留一條。