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mysql行转列,列转行

mysql行转列,列转行

作者: 尘埃里的玄 | 来源:发表于2021-12-07 16:29 被阅读0次

    一、行转列

    即将原本同一列下多行的不同内容作为多个字段,输出对应内容。

    建表语句

    DROP TABLE IF EXISTS tb_score;
    
    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;
    

    插入数据

    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);
    

    查询数据表中的内容(即转换前的结果)

    SELECT * FROM tb_score
    
    image.png
    转换后的结果:
    image.png
    可以看出,这里行转列是将原来的subject字段的多行内容选出来,作为结果集中的不同列,并根据userid进行分组显示对应的score。
    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.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
    

    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。

    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;
    

    运行结果:


    1

    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;
    
    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;
    
    image.png

    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
    
    3

    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:group_concat()函数可以很好的建属于同一分组的多个行转化为一个列。
    三、列转行

    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;
    

    插入数据

    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);
    

    查询数据表中的内容(即转换前的结果)


    image.png

    转换后:


    image.png
    本质是将userid的每个科目分数分散成一条记录显示出来。

    直接上SQL:

    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
    

    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;
    参考网址:https://www.cnblogs.com/xiaoxi/p/7151433.html

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