MySQL行列互转

sudo service mysql start --启动mysql服务
mysql -u root  --使用root用户登录
quit /exit   --退出
set names utf8;  --设置传输编码,避免中文乱码
source d:/xxx.sql; --source 文件路径
 create table tb_score(
    id int(11) not null auto_increment,
    userid varchar(20) not null,
    subject varchar(20),
    score double,
    primary key(id))
    engine=innodb default charset=utf8;
tab1:
INSERT INTO tb_score(userid,subject,score) VALUES ('001','chinese',90);
INSERT INTO tb_score(userid,subject,score) VALUES ('001','math',92);
INSERT INTO tb_score(userid,subject,score) VALUES ('001','english',80);
INSERT INTO tb_score(userid,subject,score) VALUES ('002','chinese',88);
INSERT INTO tb_score(userid,subject,score) VALUES ('002','math',90);
INSERT INTO tb_score(userid,subject,score) VALUES ('002','english',75.5);
INSERT INTO tb_score(userid,subject,score) VALUES ('003','chinese',70);
INSERT INTO tb_score(userid,subject,score) VALUES ('003','math',85);
INSERT INTO tb_score(userid,subject,score) VALUES ('003','english',90);
INSERT INTO tb_score(userid,subject,score) VALUES ('003','cs',82);
tab2:
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);

1.mysql行转列
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。

原表.png

转换后.png

a.
select userid ,
 sum(case subject when 'chinese' then score else 0 end) as chinese, 
sum(case subject when 'math' then score else 0 end ) as math ,
sum(case subject when 'english' then score else 0 end) as english ,
sum(case subject when 'cs' then score else 0 end) as cs 
from tb_score group by userid;
b.
select userid, 
sum(if(subject='chinese',score,0))as chinese, 
sum(if(subject='math',score,0))as math, 
sum(if(subject='english',score,0))as english, 
sum(if(subject='cs',score,0)) as cs 
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="chinese"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='chinese' 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​​='chinese',score,0) 作为条件，即对所有subject='chinese'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。
2.列转行

原表.png

转换后.png

select userid,'chinese' as course,cn_score as score from tb_score2 
union all 
select userid,'math' as course,math_score as score from tb_score2 
union all 
select userid,'english' as course,en_score as score from tb_score2 
union all 
select userid,'cs' as course ,cs_score as score from tb_score2 
order by userid;

注意点：UNION与UNION ALL的区别

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；

3.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='chinese',score,0))as chinese, 
sum(if(subject='math',score,0)) as math, 
sum(if(subject='english',score,0))as english, 
sum(if(subject='cs',score,0))as cs, 
sum(score) as total 
from tb_score group by userid;

行求和

select 'total', 
sum(if(subject='chinese',score,0))as chinese, 
sum(if(subject='math',score,0)) as math, 
sum(if(subject='english',score,0))as english, 
sum(if(subject='cs',score,0))as cs, 
sum(score) 
from tb_score ;

列求和

4.利用SUM(IF()) 生成列，with rollup直接生成结果

select  ifnull (userid,'total'), 
sum(if(subject='chinese',score,0))as chinese, 
sum(if(subject='math',score,0)) as math, 
sum(if(subject='english',score,0))as english, 
sum(if(subject='cs',score,0))as cs, 
sum(score) 
from tb_score 
group by userid with rollup;