【sql】部門最高工資 Department Highest Salary

問題：

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

解決：

① 讓兩個表內交起來，而後將結果表須要的列都標明，而後就是要找最高的薪水，咱們用Max關鍵字來實現。2299 ms

SELECT d.Name Department,e1.Name Employee,e1.Salary Salary
FROM Employee e1 JOIN Department d ON e1.DepartmentId = d.Id
WHERE e1.Salary IN
(SELECT MAX(Salary) FROM Employee e2 WHERE e1.DepartmentId = e2.DepartmentId);/*不能用GROUP BY 對其進行分組。。。*/

② 不用Join關鍵字，直接用Where將兩表連起來。1526 ms

SELECT d.Name Department,e1.Name Employee,e1.Salary Salary
FROM Employee e1,Department d
WHERE e1.DepartmentId = d.Id 
AND e1.Salary = 
(SELECT MAX(Salary) FROM Employee e2 WHERE e2.DepartmentId = d.Id);/*括號內的條件爲e1時，獲得的是一組數，爲d時，獲得的是一個值*/

③ 不用MAX方法而是用>=。 1187 ms

SELECT d.Name AS Department, e.Name AS Employee, e.Salary
FROM Employee e, Department d
WHERE e.DepartmentId = d.Id 
AND e.Salary >= ALL (SELECT Salary FROM Employee e2 WHERE e2.DepartmentId = d.Id);/*使用ALL是由於子查詢語句會返回不止一行數據,使用ALL會與返回的每個數據進行比較*/

SELECT d.Name Department, e1.Name Employee, e1.Salary
FROM Employee e1, Department d
WHERE e1.DepartmentId = d.Id 
AND e1.Salary >= ALL(SELECT Salary FROM Employee e2 WHERE e2.DepartmentId = e1.DepartmentId);