MySQL之两个字段的in

长见识!

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 |
±-----------±---------±-------+

因为 Employee 表包含 Salary 和 DepartmentId 字段，
我们可以以此在部门内查询最高工资。
然后，我们可以把表 Employee 和 Department 连接，
再在这张临时表里用 IN 语句查询部门名字和工资的关系。
SELECT
    Department.name AS 'Department',
    Employee.name AS 'Employee',
    Salary
FROM
    Employee
        JOIN
    Department ON Employee.DepartmentId = Department.Id
WHERE
    (Employee.DepartmentId , Salary) IN  
    (   SELECT
            DepartmentId, MAX(Salary)
        FROM
            Employee
        GROUP BY DepartmentId 
	)

链接：https://leetcode-cn.com/problems/department-highest-salary/solution/bu-men-gong-zi-zui-gao-de-yuan-gong-by-leetcode/