mysql 行转列 列转行分析

你猜 阅读:275 2020-02-17 20:55:54 评论:0

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

先来看一下转换后的结果:

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:

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

运行结果:

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;

运行结果:

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;

运行结果:

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

运行结果:

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

建表语句:

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)

SELECT * FROM tb_score1

转换后:

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;

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