Mysql 開窗函數實戰

Mysql 開窗函數在Mysql8.0+ 中可以得以使用，實在且好用。

• row number() over
• rank() over
• dense rank()
• ntile() 

我們先上測試數據，是不同姓名，不同課程的分數表；

/*測試數據*/
CREATE TABLE `school_score` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` char(1) DEFAULT NULL,
    `course` char(10) DEFAULT NULL,
  `score`  int (2) DEFAULT NULL,
  PRIMARY KEY (`id`)
) ;

INSERT INTO `test`.`school_score`(`id`, `name`,`course`,`score`) VALUES (1, 'A','Chinese',80);
INSERT INTO `test`.`school_score`(`id`, `name`,`course`,`score`) VALUES (2, 'B','Chinese',90);
INSERT INTO `test`.`school_score`(`id`, `name`,`course`,`score`) VALUES (3, 'C','Chinese',70);
INSERT INTO `test`.`school_score`(`id`, `name`,`course`,`score`) VALUES (4, 'A','Math',70);
INSERT INTO `test`.`school_score`(`id`, `name`,`course`,`score`) VALUES (5, 'B','Math',100);
INSERT INTO `test`.`school_score`(`id`, `name`,`course`,`score`) VALUES (6, 'C','Math',80);
INSERT INTO `test`.`school_score`(`id`, `name`,`course`,`score`) VALUES (7, 'A','English',90);
INSERT INTO `test`.`school_score`(`id`, `name`,`course`,`score`) VALUES (8, 'B','English',85);
INSERT INTO `test`.`school_score` (`id`, `name`,`course`,`score`) VALUES (9, 'C','English',99);

 

1. row number() over

/*開窗函數和排名類函數結合，看每個課程的排名*/

SELECT
    `name`,
    `course`,
    `score`,
    row_number ( ) over ( PARTITION BY `course` ORDER BY score DESC ) AS score_rank 
FROM
    `test`.`school_score`;

結果👇：

 

    /*使用開窗函數計算每個課程分數最高的一個*/
    
SELECT
    * 
FROM
    ( SELECT `name`, `course`, `score`, row_number ( ) over ( PARTITION BY `course` ORDER BY score DESC ) AS score_rank FROM `test`.`school_score` ) AS a 
WHERE
    a.score_rank = 1;

結果👇：

 

 

 

 

 

    /*第二部分：開窗函數和SUM() ，AVG() 等聚合函數結合*/
    
SELECT
    `name`,
    `course`,
    `score`,
    SUM( score ) over ( PARTITION BY `course` ) AS course_score_total ,
    round(AVG(score) over (PARTITION BY `course`),2)  as  course_score_avg
FROM
    `test`.`school_score`;

結果👇：

/* SUM(score) over (PARTITION BY `course` ORDER BY score ASC)   如果執行這個語句，就是在每個
課程對分數進行累加*/

SELECT
    `name`,
    `course`,
    `score`,
    SUM(score) over (PARTITION BY `course` ORDER BY score ASC ) as course_score_total
FROM
`test`.`school_score`;

 

 

 

思考🤔： 有order by ,按照排序連續累加；無order by ,計算partition by 后的和；over() 中沒有partition by ,計算所有數據總和

同時，order by 的asc 和 desc 的排序不同，有order by 的結果也不一樣。

 

2. row number() over , rank() over ,dense rank() 三者對比。

create table students_score(
    id int(4)  auto_increment primary key,
    name varchar(50) not null, 
    score int(4) not null
    );
    
    insert into students_score(name,score) values
    ('A', 300),
    ('B', 240),
    ('C', 250), 
    ('D', 280), 
    ('E', 240), 
    ('F', 200);
    

 

執行👇語句：

SELECT
    `id`,
    `name`,
    rank ( ) over ( ORDER BY score DESC ) AS r,
    DENSE_RANK ( ) OVER ( ORDER BY score DESC ) AS dense_r,
    row_number ( ) OVER ( ORDER BY score DESC ) AS row_r 
FROM
    students_score;

 

 👆 看圖🤔區別，就可以知道三者的排名的區別了，如果我是校長，我希望可以按照 DENSE_RANK() 的排序，公平且可以激勵着一代代莘莘學子。

 

3.ntile()分組

SELECT
    `id`,
    `name`,
 score,
 ntile(3) over (order by score desc)  as n
FROM
    students_score;

 

 今天的開窗函數就學習到這里，后期我會更新 Clickhouse 的類似窗口函數。