Employee 表：
+-------------+------+
| Column Name | Type |
+-------------+------+
| id | int |
| salary | int |
+-------------+------+
id 是这个表的主键。
表的每一行包含员工的工资信息。编写一个 SQL 查询，获取并返回 Employee 表中第二高的薪水 。如果不存在第二高的薪水，查询应该返回 null 。
查询结果如下例所示。
示例 1：
输入：
Employee 表：
+----+--------+
| id | salary |
+----+--------+
| 1 | 100 |
| 2 | 200 |
| 3 | 300 |
+----+--------+
输出：
+---------------------+
| SecondHighestSalary |
+---------------------+
| 200 |
+---------------------+示例 2：
输入：
Employee 表：
+----+--------+
| id | salary |
+----+--------+
| 1 | 100 |
+----+--------+
输出：
+---------------------+
| SecondHighestSalary |
+---------------------+
| null |
+---------------------+解题：
select ifnull((select distinct salary from Employee order by salary desc limit 1 offset 1), null) as SecondHighestSalary;官方解答：
1.使用子查询和limit子句
select (select distinct salary from Employee order by salary desc limit 1 offset 1) as SecondHighestSalary;2.使用IFNULL和LIMIT 子句
SELECT IFNULL((SELECT DISTINCT Salary FROM Employee ORDER BY Salary DESC LIMIT 1 OFFSET 1), NULL) AS SecondHighestSalary;