本教程操作环境：windows10系统、mysql8.0.22版本、Dell G3电脑。
mysql的行怎么转为列
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 totalFROM( 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 useridWITH 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_scoreGROUP BY useridUNIONSELECT "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_scoreGROUP 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_scoreGROUP 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。
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