mysql 行转列和列转行实例详解

你的名字 2023-02-25 02:19 50阅读 0赞

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**mysql行转列、列转行** 语句不难，不做多余解释了，看语句时，从内往外一句一句剖析。

**一、行转列**

**1.准备工作（数据整理）**

CREATE TABLE tb_score(
        id INT(11) NOT NULL auto_increment,
        userid VARCHAR(20) NOT NULL COMMENT '用户id',
        subject VARCHAR(20) COMMENT '科目',
        score DOUBLE COMMENT '成绩',
        PRIMARY KEY(id)
    )ENGINE = INNODB DEFAULT CHARSET = utf8;
    
    
    
    INSERT INTO tb_score(userid,subject,score) VALUES ('001','语文',90);
    INSERT INTO tb_score(userid,subject,score) VALUES ('001','数学',92);
    INSERT INTO tb_score(userid,subject,score) VALUES ('001','英语',80);
    INSERT INTO tb_score(userid,subject,score) VALUES ('002','语文',88);
    INSERT INTO tb_score(userid,subject,score) VALUES ('002','数学',90);
    INSERT INTO tb_score(userid,subject,score) VALUES ('002','英语',75.5);
    INSERT INTO tb_score(userid,subject,score) VALUES ('003','语文',70);
    INSERT INTO tb_score(userid,subject,score) VALUES ('003','数学',85);
    INSERT INTO tb_score(userid,subject,score) VALUES ('003','英语',90);
    INSERT INTO tb_score(userid,subject,score) VALUES ('003','政治',82);

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzM0NzU1NzY2_size_16_color_FFFFFF_t_70][]

**2.行转列实例讲解：**

**2.1、使用case...when....then 进行行转列**

SELECT userid,
    	SUM(CASE `subject` WHEN '语文' THEN score ELSE 0 END) as '语文',
    	SUM(CASE `subject` WHEN '数学' THEN score ELSE 0 END) as '数学',
    	SUM(CASE `subject` WHEN '英语' THEN score ELSE 0 END) as '英语',
    	SUM(CASE `subject` WHEN '政治' THEN score ELSE 0 END) as '政治' 
    FROM tb_score 
    GROUP BY userid

**2.2  使用IF() 进行行转列：**

SELECT userid,
    	SUM(IF(`subject`='语文',score,0)) as '语文',
    	SUM(IF(`subject`='数学',score,0)) as '数学',
    	SUM(IF(`subject`='英语',score,0)) as '英语',
    	SUM(IF(`subject`='政治',score,0)) as '政治' 
    FROM tb_score 
    GROUP BY userid

![20200710105532115.png][]

注意点：

（1）SUM() 是为了能够使用GROUP BY根据userid进行分组，因为每一个userid对应的subject="语文"的记录只有一条，所以SUM() 的值就等于对应那一条记录的score的值。

假如userid ='001' and subject='语文' 的记录有两条，则此时SUM() 的值将会是这两条记录的和，同理，使用Max()的值将会是这两条记录里面值最大的一个。但是正常情况下，一个user对应一个subject只有一个分数，因此可以使用SUM()、MAX()、MIN()、AVG()等聚合函数都可以达到行转列的效果。

（2）IF(subject='语文',score,0) 作为条件，即对所有subject='语文'的记录的score字段进行SUM()、MAX()、MIN()、AVG()操作，如果score没有值则默认为0。

**  2.3 利用SUM(IF()) 生成列 + WITH ROLLUP 生成汇总行,并利用 IFNULL将汇总行标题显示为Total**

SELECT IFNULL(userid,'total') AS userid,
    SUM(IF(`subject`='语文',score,0)) AS 语文,
    SUM(IF(`subject`='数学',score,0)) AS 数学,
    SUM(IF(`subject`='英语',score,0)) AS 英语,
    SUM(IF(`subject`='政治',score,0)) AS 政治,
    SUM(IF(`subject`='total',score,0)) AS total
    FROM(
        SELECT userid,IFNULL(`subject`,'total') AS `subject`,SUM(score) AS score
        FROM tb_score
        GROUP BY userid,`subject`
        WITH ROLLUP
        HAVING userid IS NOT NULL
    )AS A 
    GROUP BY userid
    WITH ROLLUP;

![20200710105758114.png][]

**2.4 利用SUM(IF()) 生成列 + UNION 生成汇总行,并利用 IFNULL将汇总行标题显示为 Total**

SELECT userid,
    	SUM(IF(`subject`='语文',score,0)) AS 语文,
    	SUM(IF(`subject`='数学',score,0)) AS 数学,
    	SUM(IF(`subject`='英语',score,0)) AS 英语,
    	SUM(IF(`subject`='政治',score,0)) AS 政治,
    	SUM(score) AS TOTAL 
    FROM tb_score
    GROUP BY userid
    UNION
    SELECT 'TOTAL',SUM(IF(`subject`='语文',score,0)) AS 语文,
    	SUM(IF(`subject`='数学',score,0)) AS 数学,
    	SUM(IF(`subject`='英语',score,0)) AS 英语,
    	SUM(IF(`subject`='政治',score,0)) AS 政治,
    	SUM(score) FROM tb_score

![20200710110030347.png][]

**2.5 利用SUM(IF()) 生成列，直接生成结果不再利用子查询**

SELECT IFNULL(userid,'TOTAL') AS userid,
    	SUM(IF(`subject`='语文',score,0)) AS 语文,
    	SUM(IF(`subject`='数学',score,0)) AS 数学,
    	SUM(IF(`subject`='英语',score,0)) AS 英语,
    	SUM(IF(`subject`='政治',score,0)) AS 政治,
    	SUM(score) AS TOTAL 
    FROM tb_score
    GROUP BY userid WITH ROLLUP;

![20200710110916911.png][]

**2.6 动态，适用于列不确定情况**

SET @EE='';
    select @EE :=CONCAT(@EE,'sum(if(subject= \'',subject,'\',score,0)) as ',subject, ',') AS aa FROM (SELECT DISTINCT subject FROM tb_score) A ;
     
    SET @QQ = CONCAT('select ifnull(userid,\'TOTAL\')as userid,',@EE,' sum(score) as TOTAL from tb_score group by userid WITH ROLLUP');
    -- SELECT @QQ;
     
    PREPARE stmt FROM @QQ;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;

![20200710111115905.png][]

![2020071011113885.png][]

**2.7 合并字段显示：利用group\_concat()**

SELECT userid,GROUP_CONCAT(`subject`,":",score)AS 成绩 FROM tb_score GROUP BY userid

![20200710111356754.png][]

group_concat()，手册上说明:该函数返回带有来自一个组的连接的非NULL值的字符串结果。
    比较抽象，难以理解。通俗点理解，其实是这样的：group_concat()会计算哪些行属于同一组，将属于同一组的列显示出来。要返回哪些列，由函数参数(就是字段名)决定。分组必须有个标准，就是根据group by指定的列进行分组。
    
    结论：groupconcat()函数可以很好的建属于同一分组的多个行转化为一个列。

**二、列转行实例讲解：**

**1.  准备工作（数据整理）**

CREATE TABLE tb_score1(
        id INT(11) NOT NULL auto_increment,
        userid VARCHAR(20) NOT NULL COMMENT '用户id',
        cn_score DOUBLE COMMENT '语文成绩',
        math_score DOUBLE COMMENT '数学成绩',
        en_score DOUBLE COMMENT '英语成绩',
        po_score DOUBLE COMMENT '政治成绩',
        PRIMARY KEY(id)
    )ENGINE = INNODB DEFAULT CHARSET = utf8;
    
    
    INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES ('001',90,92,80,0);
    INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES ('002',88,90,75.5,0);
    INSERT INTO tb_score1(userid,cn_score,math_score,en_score,po_score) VALUES ('003',70,85,90,82);

![20200710112128538.png][]

**2.  列转行实例讲解**

SELECT userid,'语文' AS course,cn_score AS score FROM tb_score1
    UNION ALL
    SELECT userid,'数学' AS course,math_score AS score FROM tb_score1
    UNION ALL
    SELECT userid,'英语' AS course,en_score AS score FROM tb_score1
    UNION ALL
    SELECT userid,'政治' AS course,po_score AS score FROM tb_score1
    ORDER BY userid

![20200710112336286.png][]

这里将每个userid对应的多个科目的成绩查出来，通过UNION ALL将结果集加起来，达到上图的效果。

附：UNION与UNION ALL的区别（摘）：

1.对重复结果的处理：UNION会去掉重复记录，UNION ALL不会；

2.对排序的处理：UNION会排序，UNION ALL只是简单地将两个结果集合并；

3.效率方面的区别：因为UNION 会做去重和排序处理，因此效率比UNION ALL慢很多；

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[20200710105532115.png]: /images/20230209/4552bcf74e8a47abb3d63f67c18e47e4.png
[20200710105758114.png]: /images/20230209/97fdb4c4730a4233b74cd2a483c05f3d.png
[20200710110030347.png]: /images/20230209/4a16439196c84f2c92d7a969188f59cb.png
[20200710110916911.png]: /images/20230209/456b8f60236541a8957238756b331bbd.png
[20200710111115905.png]: /images/20230209/92babd79a7254e81a83337d597996656.png
[2020071011113885.png]: https://img-blog.csdnimg.cn/2020071011113885.png
[20200710111356754.png]: https://img-blog.csdnimg.cn/20200710111356754.png
[20200710112128538.png]: https://img-blog.csdnimg.cn/20200710112128538.png
[20200710112336286.png]: https://img-blog.csdnimg.cn/20200710112336286.png