Mysql中行转列和列转行

我会带着你远行 2024-04-17 05:30 56阅读 0赞

**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：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**

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；  
————————————————  
版权声明：本文为CSDN博主「天问运」的原创文章，遵循CC 4.0 by-sa版权协议，转载请附上原文出处链接及本声明。  
原文链接：https://blog.csdn.net/lilong329329/article/details/81664451