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

非常常見的資料庫查詢問題，以部門為單位求最高工資。 上傳這道題的目的是為提高對查詢語句的理解： 先公佈解法：

SELECT d.Name AS Department, e1.Name AS
 
 Employee, e1.Salary FROM Employee e1
JOIN Department d ON e1.DepartmentId = d.Id WHERE Salary =
(SELECT MAX(Salary) FROM Employee e2 WHERE e1.DepartmentId = e2.DepartmentId);

對網上這種解法一開始並不能看懂，因為固定思維on後面有一個=連線的條件，總是會讓我想到結果集只有一行資料。然而並不是如此，首先，select語句可以看成是迴圈語句，遍歷表中的每一行資料並篩選出符合條件的，如果這樣理解，那麼=號後面子查詢的結果會隨遍歷而變化，當指標指到並列後表中的第一條資料時，子查詢結果為90000，（因為此時DepartmentId為1）.因此第一條資料不符合條件。以此類推能夠得到正確結果。 我的第一次解法：

SELECT d.Name AS Department, e1.Name AS Employee, MAX(e1.Salary) Salary FROM Employee e1
JOIN Department d ON e1.DepartmentId = d.Id GROUP BY Department;

看似沒有什麼問題，按部門為單位求最大值，因為之前按部門為單位求和時就是這樣寫的。兩者的區別為：按部門求和問題最終資料是所屬於部門的，而最大值是所屬於部門中的某人的，這樣執行的結果就是工資和員工不匹配，因為MAX只負責求某列最大值，這樣執行的結果是最大值正確無誤而對應的員工則是部門中編號靠前的第一個員工，因為並沒有條件限制員工。剩下的被捨棄，因為MAX只有一行資料。 總結： select語句的正確理解和group by 的適當運用。