Employee 表包含所有员工信息，每个员工有其对应的 Id, salary 和 department Id。
+----+-------+--------+--------------+
| Id | Name | Salary | DepartmentId |
+----+-------+--------+--------------+
| 1 | Joe | 70000 | 1 |
| 2 | Henry | 80000 | 2 |
| 3 | Sam | 60000 | 2 |
| 4 | Max | 90000 | 1 |
+----+-------+--------+--------------+
Department 表包含公司所有部门的信息。
+----+----------+
| Id | Name |
+----+----------+
| 1 | IT |
| 2 | Sales |
+----+----------+
编写一个 SQL 查询，找出每个部门工资最高的员工。例如，根据上述给定的表格，Max 在 IT 部门有最高工资，Henry 在 Sales 部门有最高工资。
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT | Max | 90000 |
| Sales | Henry | 80000 |
+------------+----------+--------+
解法一：
SELECT Department.Name as Department, Employee.Name as Employee, MAX(Employee.Salary) as Salary FROM Employee LEFT JOIN Department ON Employee.DepartmentId = Department.Id GROUP BY Employee.DepartmentId;
输入：
{"headers": {"Employee": ["Id", "Name", "Salary", "DepartmentId"], "Department": ["Id", "Name"]}, "rows": {"Employee": [[1, "Joe", 70000, 1], [2, "Jim", 90000, 1], [3, "Henry", 80000, 2], [4, "Sam", 60000, 2], [5, "Max", 90000, 1]], "Department": [[1, "IT"], [2, "Sales"]]}}
输出：
{"headers":["Department","Employee","Salary"],"values":[["IT","Joe",90000],["Sales","Henry",80000]]}
预期：
{"headers":["Department","Employee","Salary"],"values":[["IT","Jim",90000],["Sales","Henry",80000],["IT","Max",90000]]}
没有达到预期结果，原因group by 进行了去重，如果有同部门薪水想同多个员工得不到预期结果。
解法二：
select d.Name as Department,e.Name as Employee ,e.Salary from Employee e join Department d on e.DepartmentId = d.Id where (e.DepartmentId ,e.Salary) in (select DepartmentId ,max(Salary) as Salary from Employee group by DepartmentId);
正确。
先根据部门DepartmentId group by 出部门id与最高的薪水值作为筛选条件。
select DepartmentId ,max(Salary) as Salary from Employee group by DepartmentId
在两表left join 根据得出的条件筛选满足DepartmentId 与 Salary 最大值的员工。
Topic Source: stay button (LeetCode)
link: https: //leetcode-cn.com/problems/department-highest-salary
