MySql中的行专列，列转行

原文链接： https://blog.csdn.net/lilong329329/article/details/81664451

一、行转列

行转列就是同一列下的多行不同数据一行显示

1，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;

2.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);

3.查询出来的数据格式(转换之前)

转换之后的结果

这样的需要还是蛮常见的，是非常重要的一个sql语句功能。这里我们可以看见我们将subject的多行数据转换成了多列数据，实现了行转列

sql实现功能

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

（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;

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

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;

6.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

2.行转列

建表

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);

查询结果(转换前)

SELECT * FROM tb_score1

目的是可以将userid的每一门成绩以每一行的形式显示出来
目的结果:

sql实现

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

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；

原文链接：https://blog.csdn.net/lilong329329/article/details/81664451