LeetCode Database #176 Second Highest Salary

LeetCode Database #176 Second Highest Salary

个人比较喜欢小写SQL代码，反正可以写完再格式化

题目内容

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 query should return

200

as the second highest salary. If there is no second highest salary, then the query should return

null

.

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

解题

选出第二大的薪水，肯定要做的是排序

order by Salary desc

，再使用

limit 1 offset 1

来选出第二大的（这个不看solution我还真不知道，数据库还是学的太少）

select distinct Salary as SecondHighestSalary
from Employee
order by Salary desc
limit 1 offset 1;

但是直接这样提交是错误的，因为题目要求当没有结果时返回

null

，而空集并不等于

null

，参考solution，有两种方法。

第一种是使用

ifnull(expr1, expr2)

函数，如果

expr1

不为

null

返回

expr1

，否则返回

expr2

。所以可以写作

select ifnull((
select distinct Salary
from Employee
order by Salary desc
limit 1 offset 1),
null) as SecondHighestSalary;

第二种比较骚，

select

一个空集的结果为

null

。写作

select (
select distinct Salary
from Employee
order by Salary desc
limit 1 offset 1) as SecondHighestSalary;