【DATAWHALE 第六章：决胜秋招】

一、练习一：各部门工资最高的员工（难度：中等）

1.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

create table employee(
	employee_id  char(4) not null,
    employee_name varchar(100) not null,
    salary integer not null,
    department_id char(4) not null,
    primary key(Employee_id)
);

insert into employee values('0001','Joe','70000','0001');
insert into employee values('0002','Henry','80000','0002');
insert into employee values('0003','Sam','60000','0002');
insert into employee values('0004','Max','50000','0001');

2.创建Department 表，包含公司所有部门的信息。

create table department(
	department_id char(4) not null,
		department_name varchar(100) not null,
        primary key(department_id)
);

insert into department values('0001','IT');
insert into department values('0002','Sales');

3.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。

select employee_name, max(salary) as max_salary from employee 
group by department_id

练习二: 换座位（难度：中等）

1.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。

（1）创建表格：

create table seat(id int(11) not null,
					student varchar(100) not null,
					primary key(id));
-- 插入数据
insert into seat(id,student) values('1','ABBot');
insert into seat(id,student) values('2','Doris');
insert into seat(id,student) values('3','Emerson');
insert into seat(id,student) values('4','Green');
insert into seat(id,student) values('5','Jeames');

（2）改变相邻俩学生的座位：

SELECT
    (CASE
        WHEN MOD(id, 2) != 0 AND counts != id THEN id + 1
        WHEN MOD(id, 2) != 0 AND counts = id THEN id
        ELSE id - 1                                        
    END) AS id,
    student
FROM
    seat,
    (SELECT
        COUNT(*) AS counts
    FROM
        seat) AS seat_counts
ORDER BY id ASC;