# 176. Second Highest Salary

> Nguồn: https://tiennhm.io.vn/docs/leetcode/database/176-second-highest-salary
> Write a solution to find the second highest salary from the `Employee` table. If there is no second highest salary, return `null` (return `None` in Pandas).

## Giới thiệu bài toán

Chi tiết: https://leetcode.com/problems/second-highest-salary/

Table: `Employee`

| Column Name | Type |
| ----------- | ---- |
| id          | int  |
| salary      | int  |

- `id` is the primary key (column with unique values) for this table.
- Each row of this table contains information about the salary of an employee.

> **Tip: Yêu cầu**
>
> Write a solution to find the second highest salary from the `Employee` table. If there is no second highest salary, return `null` (return `None` in Pandas).
>
> The result format is in the following example.
**Example 1:**

- Input: `Employee` table:

| id | salary |
| -- | ------ |
| 1  | 100    |
| 2  | 200    |
| 3  | 300    |

- Output:

| SecondHighestSalary |
| ------------------- |
| 200                 |

**Example 2:**

- Input: `Employee` table:

| id | salary |
| -- | ------ |
| 1  | 100    |

- Output:

| SecondHighestSalary |
| ------------------- |
| null                |

## Giải quyết bài toán

```sql
SELECT (
    SELECT DISTINCT(salary)
    FROM Employee
    ORDER BY salary DESC
    LIMIT 1 OFFSET 1
) AS SecondHighestSalary;
```

Tham khảo: https://leetcode.com/submissions/detail/1038480623/