Employee 表：
+-------------+------+
| Column Name | Type |
+-------------+------+
| id | int |
| salary | int |
+-------------+------+
id 是這個表的主鍵。
表的每一行包含員工的工資資訊。
 

編寫一個 SQL 查詢，獲取並返回 Employee 表中第二高的薪水 。如果不存在第二高的薪水，查詢應該返回 null 。

查詢結果如下例所示。

示例 1：

輸入：
Employee 表：
+----+--------+
| id | salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+
輸出：
+---------------------+
| SecondHighestSalary |
+---------------------+
| 200 |
+---------------------+
示例 2：

輸入：
Employee 表：
+----+--------+
| id | salary |
+----+--------+
| 1 | 100 |
+----+--------+
輸出：
+---------------------+
| SecondHighestSalary |
+---------------------+
| null |
+---------------------+

來源：力扣（LeetCode）

1.

SELECT DISTINCT salary AS secondHighestSalary FROM Employee ORDER BY salary DESC
 
 LIMIT 1,1

2.

SELECT salary AS secondHighestSalary FROM Employee ORDER BY salary DESC LIMIT 1 OFFSET 1