[176\. 第二高的薪水](https://leetcode-cn.com/problems/second-highest-salary/)
===
發奮圖強!!!!
### 方法一
```
SELECT DISTINCT
Salary as SecondHighestSalary
FROM Employee
ORDER BY Salary DESC
LIMIT 1 OFFSET 1
```
salary DESC LIMIT 1 OFFSET 1
but 如果只有一條數據數據了???
gg
```
SELECT (
SELECT DISTINCT
Salary as SecondHighestSalary
FROM Employee
ORDER BY Salary DESC
LIMIT 1 OFFSET 1
) AS SecondHighestSalary
```
### 方法二：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
```
IFNULL的作用是什么？下面一個簡單的sql和結果說明，如果IFNULL(a,b)，a接收的值為null，則返回b，否則返回a；
