Datawhale MySQL 训练营 Task5

数据导入导出

• 导入table
  http://www.runoob.com/mysql/mysql-database-import.html

• 导出table
  http://www.runoob.com/mysql/mysql-database-export.html

SELECT * FROM runoob_tbl
INTO OUTFILE '/tmp/runoob.txt'

设置分隔符

SELECT a,b,a+b INTO OUTFILE '/tmp/result.text'
FIELDS TERMINATED BY ',' OPTIONALLY ENCLOSED BY '"'
LINES TERMINATED BY '\n'
FROM test_table;

作业

• 1.项目七: 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    |
+----+----------+

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

解答：

CREATE TABLE employee(
Id INT NOT NULL PRIMARY KEY,
Name VARCHAR(20) NOT NULL,
Salary INT,
DepartmentId INT
);

INSERT INTO employee
(Id,Name,Salary,DepartmentId)
VALUES
(1,'Joe',70000,1),
(2,'Henry',80000,2),
(3,'Sam',60000,2),
(4,'Max',90000,2);

CREATE TABLE Department(
Id INT NOT NULL,
Name VARCHAR(20)
);

INSERT INTO Department
(Id,Name)
VALUES
(1,'IT'),
(2,'Sales');

SELECT Department.Name AS Department, employee.Name As Employee, employee.Salary AS Salary
FROM Department
JOIN employee
WHERE Department.Id = employee.DepartmentId
GROUP BY employee.DepartmentId
HAVING MAX(Salary)

结果：

• 2.项目八: 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。

解答：

CREATE TABLE seat(
id INT NOT NULL PRIMARY KEY,
student VARCHAR(20)
);

INSERT INTO seat
VALUES
(1,'Abbot'),
(2,'Doris'),
(3,'Emerson'),
(4,'Green'),
(5,'Jeames');

SELECT s.id,s.student from (
    SELECT id-1 as id, student
    FROM seat
    WHERE id%2 = 0

    UNION

    SELECT id+1 as id, student
    FROM seat
    WHERE id%2 =1 AND id != (SELECT MAX(id) FROM seat)

    UNION 

    SELECT id,student
    FROM seat
    WHERE id%2=1 AND id = (SELECT MAX(id) FROM seat);
)s
ORDER BY s.id;

结果：

• 3.项目九: 分数排名（难度：中等）
  编写一个 SQL 查询来实现分数排名。如果两个分数相同，则两个分数排名（Rank）相同。请注意，平分后的下一个名次应该是下一个连续的整数值。换句话说，名次之间不应该有“间隔”。
  创建以下score表：

+----+-------+
| Id | Score |
+----+-------+
| 1  | 3.50  |
| 2  | 3.65  |
| 3  | 4.00  |
| 4  | 3.85  |
| 5  | 4.00  |
| 6  | 3.65  |
+----+-------+

例如，根据上述给定的 Scores 表，你的查询应该返回（按分数从高到低排列）：

+-------+------+
| Score | Rank |
+-------+------+
| 4.00  | 1    |
| 4.00  | 1    |
| 3.85  | 2    |
| 3.65  | 3    |
| 3.65  | 3    |
| 3.50  | 4    |
+-------+------+

CREATE TABLE score(
id int,
score DECIMAL(3,2)
);

INSERT INTO score
VALUES
(1,3.50),
(2,3.65),
(3,4.00),
(4,3.85),
(5,4.00),
(6,3.65);

SELECT id, score,
CASE
WHEN @pre_score = score THEN @rank_tmp
WHEN @pre_score:=score THEN @rank_tmp:=@rank_tmp+1
END AS 'rank'
FROM score,(SELECT @rank_tmp:=0,@pre_score:=NULL) tmp
ORDER BY
score.score DESC;

结果：

这里用了变量和 CASE WHEN 语句，十分巧妙。
在MySQL 8.x 版本引入了窗口函数，包括 rank()、dense_rank()；所以可以这样写

SELECT
    id,
    score,
    dense_rank ( ) over ( ORDER BY score.score DESC ) AS 'rank'
FROM
    score
ORDER BY
    score DESC;

轻松优雅美滋滋。

参考

• 排序部分参考了 https://www.jianshu.com/p/bb1b72a1623e
• 换座位参考了 https://blog.csdn.net/wal1314520/article/details/80117658
• 窗口函数参考了 https://blog.51cto.com/11103985/2341971