[LeetCode]Department Highest Salary,解题报告

题目

The Employee table holds all employees. Every employee has an Id, a salary, and there is also a column for the department Id.

Id	Name	Salary	DepartmentId
1	Joe	70000	1
2	Henry	80000	2
3	Sam	60000	2
4	Max	90000	1

The Department table holds all departments of the company.

Id	Name
1	IT
2	Sales

Write a SQL query to find employees who have the highest salary in each of the departments. For the above tables, Max has the highest salary in the IT department and Henry has the highest salary in the Sales department.

Department	Employee	Salary
IT	Max	90000
Sales	Henry	80000

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思路

看到这到题目，我首先考虑的是数据库级联。具体思路如下：

1. 查找每一部门的最高薪水。

select e.DepartmentId, MAX(e.Salary) as Salary, d.Name as Department from Employee as e inner join Department as d on e.DepartmentId = d.Id group by e.DepartmentId;

语句执行完成后，生成的表结构如下：

DepartmentId	Salary	Department
1	9000	IT
2	8000	Sales

2. 用上述生成的临时表和Employee表再做级联，找出题目要求的字段。

select t.Department as Department, e.Name as Employee, t.Salary as Salary from Employee as e inner join (1-sql生成的表) as t on e.Salary = t.Salary and and e.DepartmentId = t.DepartmentId;

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AC SQL

最终的ac sql语句如下：

select t.Department as Department, e.Name as Employee, t.Salary as Salary from Employee as e inner join (select e.DepartmentId, MAX(e.Salary) as Salary, d.Name as Department from Employee as e inner join Department as d on e.DepartmentId = d.Id group by e.DepartmentId) as t on e.Salary = t.Salary and e.DepartmentId = t.DepartmentId;