LeetCode（184）：部门工资最高的员工 Reformat Department Table（SQL）

2020.7.8 LeetCode 从零单刷个人笔记整理（持续更新）

github：https://github.com/ChopinXBP/LeetCode-Babel

传送门：部门工资最高的员工

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  |
+------------+----------+--------+

---

#注意：1.最高工资不止一人；2.可能有人的部门号没有对应部门名

#左连接后要判断NULL值
SELECT d.Name AS 'Department', t.Name AS 'Employee', t.Salary AS 'Salary'
FROM (
    SELECT e.Name, e.Salary, e.DepartmentId
    FROM (SELECT DepartmentId, MAX(Salary) AS 'maxSalary'
        FROM Employee
        GROUP BY DepartmentId) m, Employee e
    WHERE e.Salary = m.maxSalary AND e.DepartmentId = m.DepartmentId
) t
LEFT JOIN Department d
ON t.DepartmentId = d.Id
WHERE d.Name IS NOT NULL;

#内连接会自动去除NULL值
SELECT d.Name AS 'Department', t.Name AS 'Employee', t.Salary AS 'Salary'
FROM (
    SELECT e.Name, e.Salary, e.DepartmentId
    FROM (SELECT DepartmentId, MAX(Salary) AS 'maxSalary'
        FROM Employee
        GROUP BY DepartmentId) m, Employee e
    WHERE e.Salary = m.maxSalary AND e.DepartmentId = m.DepartmentId
) t
INNER JOIN Department d
ON t.DepartmentId = d.Id;

#IN左右可以连接多个值对
SELECT d.Name AS 'Department', e.Name AS 'Employee', e.Salary AS 'Salary'
FROM Employee e
INNER JOIN Department d
ON e.DepartmentId = d.Id
WHERE (e.DepartmentId, e.Salary) IN (
    SELECT DepartmentId, MAX(Salary)
    FROM Employee
    GROUP BY DepartmentId
);

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#Coding一小时，Copying一秒钟。留个言点个赞呗，谢谢你#