題目
SQL架構
Create table If Not Exists Employee (Id int, Name varchar(255), Salary int, DepartmentId int)
Create table If Not Exists Department (Id int, Name varchar(255))
Truncate table Employee
insert into Employee (Id, Name, Salary, DepartmentId) values ('1', 'Joe', '85000', '1')
insert into Employee (Id, Name, Salary, DepartmentId) values ('2', 'Henry', '80000', '2')
insert into Employee (Id, Name, Salary, DepartmentId) values ('3', 'Sam', '60000', '2')
insert into Employee (Id, Name, Salary, DepartmentId) values ('4', 'Max', '90000', '1')
insert into Employee (Id, Name, Salary, DepartmentId) values ('5', 'Janet', '69000', '1')
insert into Employee (Id, Name, Salary, DepartmentId) values ('6', 'Randy', '85000', '1')
insert into Employee (Id, Name, Salary, DepartmentId) values ('7', 'Will', '70000', '1')
Truncate table Department
insert into Department (Id, Name) values ('1', 'IT')
insert into Department (Id, Name) values ('2', 'Sales')
Employee 表包含所有員工信息，每個員工有其對應的 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 |
| 5 | Janet | 69000 | 1 |
| 6 | Randy | 85000 | 1 |
+----+-------+--------+--------------+
Department 表包含公司所有部門的信息。
+----+----------+
| Id | Name |
+----+----------+
| 1 | IT |
| 2 | Sales |
+----+----------+
編寫一個 SQL 查詢，找出每個部門工資前三高的員工。例如，根據上述給定的表格，查詢結果應返回：
+------------+----------+--------+
| Department | Employee | Salary |
+------------+----------+--------+
| IT | Max | 90000 |
| IT | Randy | 85000 |
| IT | Joe | 70000 |
| Sales | Henry | 80000 |
| Sales | Sam | 60000 |
+------------+----------+--------+
題解
回憶一下 count 函數
count(字段名) # 返回表中該字段總共有多少條記錄
回憶一下 DISTINCT 關鍵字
DISTINCT 字段名 # 過濾字段中的重複記錄
我們先找出公司裏前 3 高的薪水，意思是不超過三個值比這些值大
SELECT e1.Salary
FROM Employee AS e1
WHERE 3 >
(SELECT count(DISTINCT e2.Salary)
FROM Employee AS e2
WHERE e1.Salary < e2.Salary AND e1.DepartmentId = e2.DepartmentId) ;
舉個栗子：
當 e1 = e2 = [4,5,6,7,8]
e1.Salary = 4，e2.Salary 可以取值 [5,6,7,8]，count(DISTINCT e2.Salary) = 4
e1.Salary = 5，e2.Salary 可以取值 [6,7,8]，count(DISTINCT e2.Salary) = 3
e1.Salary = 6，e2.Salary 可以取值 [7,8]，count(DISTINCT e2.Salary) = 2
e1.Salary = 7，e2.Salary 可以取值 [8]，count(DISTINCT e2.Salary) = 1
e1.Salary = 8，e2.Salary 可以取值 []，count(DISTINCT e2.Salary) = 0
最後 3 > count(DISTINCT e2.Salary)，所以 e1.Salary 可取值爲 [6,7,8]，即集合中前 3 高的薪水
再把表 Department 和表 Employee 連接獲得各個部門工資前三高的員工
SELECT
Department.NAME AS Department,
e1.NAME AS Employee,
e1.Salary AS Salary
FROM
Employee AS e1,Department
WHERE
e1.DepartmentId = Department.Id
AND 3 > (SELECT count( DISTINCT e2.Salary )
FROM Employee AS e2
WHERE e1.Salary < e2.Salary AND e1.DepartmentId = e2.DepartmentId )
ORDER BY Department.NAME,Salary DESC;