mysql行转列(7种方法)和列转行的实现_Mysql

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。

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。

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