“多个item也可以用 IN , 奇怪的知识增加了”
题干
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 | Jim | 90000 | 1 | | 3 | Henry | 80000 | 2 | | 4 | Sam | 60000 | 2 | | 5 | 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。对于上述表，您的 SQL 查询应返回以下行（行的顺序无关紧要）。
+------------+----------+--------+ | Department | Employee | Salary | +------------+----------+--------+ | IT | Max | 90000 | | IT | Jim | 90000 | | Sales | Henry | 80000 | +------------+----------+--------+
解释：
Max 和 Jim 在 IT 部门的工资都是最高的，Henry 在销售部的工资最高。
题解
方法：使用 JOIN 和 IN 语句
算法
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。
SELECT
DepartmentId, MAX(Salary)
FROM
Employee
GROUP BY DepartmentId;
注意：有可能有多个员工同时拥有最高工资，所以最好在这个查询中不包含雇员名字的信息。
| DepartmentId | MAX(Salary) |
|--------------|-------------|
| 1 | 90000 |
| 2 | 80000 |
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。
MySQL
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
)
;
| Department | Employee | Salary |
|------------|----------|--------|
| Sales | Henry | 80000 |
| IT | Max | 90000 |
