LeetCode笔记:176. Second Highest Salary
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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 |+---------------------+
思考:
这里需要注意的一个条件是,当查询输出无结果时,需要返回null
。
因此,将查询结果倒序DESC
排序后使用limit 1,1
进行筛选的方法不可行,因为limit查询无结果时,返回为空。
所以其中一种办法是使用max(expr)
函数,当无结果时,会返回null
。
SELECT max(Salary) AS SecondHighestSalaryFROM EmployeeWHERE Salary < ( SELECT max(Salary) AS SecondHighestSalary FROM EMPLOYEE)
拓展:
比较聚合函数如max,min对于空数据集返回NULL,统计聚合函数如count,sum通常返回0。
——MYSQL 什么时候返回空,什么时候返回NULL,为什么这两条语句返回结果不一样? - 回答作者: Ivony https://zhihu.com/question/40540817/answer/87069604
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