SQL综合练习（一）

练习一: 各部门工资最高的员工（难度：中等）

创建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            |
+----+-------+--------+--------------+

创建Department 表，包含公司所有部门的信息。

+----+----------+
| Id | Name     |
+----+----------+
| 1  | IT       |
| 2  | Sales    |
+----+----------+

CREATE TABLE employee(
id int(11) NOT NULL,
name VARCHAR(50) NOT NULL,
salary DECIMAL(10,2) NOT NULL DEFAULT '0.00',
departmentid int(11) NOT NULL,
PRIMARY KEY (`id`)
);

INSERT INTO `employee` VALUES ('1', 'Joe', '70000.00', '1');
INSERT INTO `employee` VALUES ('2', 'Herry', '80000.00', '2');
INSERT INTO `employee` VALUES ('3', 'Sam', '60000.00', '2');
INSERT INTO `employee` VALUES ('4', 'Max', '90000.00', '1');



CREATE TABLE department(
id int(11) NOT NULL,
name VARCHAR(100) NOT NULL,
PRIMARY KEY (`id`)
);

INSERT INTO `department` VALUES ('1', 'IT');
INSERT INTO `department` VALUES ('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  |
+------------+----------+--------+

SELECT employee.*,t.name department,t.maxsal
FROM employee
INNER JOIN

(SELECT employee.departmentid, department.`name`, MAX(salary) maxsal
FROM employee
INNER JOIN department
ON employee.departmentid=department.id
GROUP BY employee.departmentid, department.`name`) AS t

ON employee.departmentid = t.departmentid

WHERE employee.salary=t.maxsal

1    IT    90000.00
2    Sales    80000.00


2    Herry    80000.00    2    Sales    80000.00
4    Max    90000.00    1    IT    90000.00

select Department.Name as Department,Employee.Name as Employee,Salary
from Employee join Department
on Employee.DepartmentId = Department.Id 
where (Salary,DepartmentId) in 
(select max(Salary),DepartmentId
from Employee
group by DepartmentId)

90000.00    1
80000.00    2

Sales    Herry    80000.00
IT    Max    90000.00

练习二: 换座位（难度：中等）

小美是一所中学的信息科技老师，她有一张 seat 座位表，平时用来储存学生名字和与他们相对应的座位 id。

其中纵列的id是连续递增的

小美想改变相邻俩学生的座位。

你能不能帮她写一个 SQL query 来输出小美想要的结果呢？

请创建如下所示seat表：

示例：

+---------+---------+
|    id   | student |
+---------+---------+
|    1    | Abbot   |
|    2    | Doris   |
|    3    | Emerson |
|    4    | Green   |
|    5    | Jeames  |
+---------+---------+

假如数据输入的是上表，则输出结果如下：

+---------+---------+
|    id   | student |
+---------+---------+
|    1    | Doris   |
|    2    | Abbot   |
|    3    | Green   |
|    4    | Emerson |
|    5    | Jeames  |
+---------+---------+

注意：
如果学生人数是奇数，则不需要改变最后一个同学的座位。

SELECT
            CASE WHEN id = (SELECT MAX(id) FROM seat) THEN id#最后一个
                     WHEN MOD(id,2)=1 THEN id+1#奇数
                     WHEN MOD(id,2)=0 THEN id-1#偶数
                     ELSE NULL END AS id,
      student
 FROM seat
ORDER BY id;

SELECT (
    CASE
        #学生人数为奇数时，学生的id加一 
        WHEN MOD(id,2)!=0 AND id!=counts THEN id+1       
        #学生人数为奇数时，最后一个学生的id不变
        WHEN MOD(id,2)!=0 AND id=counts THEN id
        ELSE id-1 
    END )AS id,student
from seat,(SELECT COUNT(*) AS counts FROM seat) AS seat_counts 
ORDER BY id;